Fix config file parsing on musl libc

On the musl libc, autorun, layout and workspace name settings were
always rejected as invalid. As it turns out, parsing those was relying
on sscanf incorrectly matching %Nc as long as there is at least one
character. This is fixed by matching only the initial part of the string
via sscanf and skipping ahead by the amount of bytes consumed. This also
avoids unnecessary zeroing, copying and possible implicit truncation.

Relevant glibc bug:
https://sourceware.org/bugzilla/show_bug.cgi?id=12701

spectrwm.c

@@ -8923,8 +8923,8 @@ int
 setconfvalue(const char *selector, const char *value, int flags)
 {
 	struct workspace	*ws;
-	int			i, ws_id, num_screens;
-	char			*b, *str, *sp, s[1024];
+	int			i, ws_id, num_screens, n;
+	char			*b, *str, *sp;
 
 	switch (flags) {
 	case SWM_S_BAR_ACTION:
@@ -9148,9 +9148,11 @@ setconfvalue(const char *selector, const char *value, int flags)
 		if (getenv("SWM_STARTED") != NULL)
 			return (0);
 
-		bzero(s, sizeof s);
-		if (sscanf(value, "ws[%d]:%1023c", &ws_id, s) != 2)
+		n = 0;
+		if (sscanf(value, "ws[%d]:%n", &ws_id, &n) != 1 || n == 0 ||
+		    value[n] == '\0')
 			errx(1, "invalid entry, should be 'ws[<idx>]:name'");
+		value += n;
 		ws_id--;
 		if (ws_id < 0 || ws_id >= workspace_limit)
 			errx(1, "setconfvalue: workspace_name: invalid "
@@ -9160,14 +9162,12 @@ setconfvalue(const char *selector, const char *value, int flags)
 		for (i = 0; i < num_screens; ++i) {
 			ws = (struct workspace *)&screens[i].ws;
 
-			if (strlen(s) > 0) {
-				free(ws[ws_id].name);
-				if ((ws[ws_id].name = strdup(s)) == NULL)
-					err(1, "setconfvalue: workspace_name.");
+			free(ws[ws_id].name);
+			if ((ws[ws_id].name = strdup(value)) == NULL)
+				err(1, "setconfvalue: workspace_name.");
 
-				ewmh_update_desktop_names();
-				ewmh_get_desktop_names();
-			}
+			ewmh_update_desktop_names();
+			ewmh_get_desktop_names();
 		}
 		break;
 	default:
@@ -9271,10 +9271,9 @@ int
 setautorun(const char *selector, const char *value, int flags)
 {
 	int			ws_id;
-	char			s[1024];
 	char			*ap, *sp, *str;
 	union arg		a;
-	int			argc = 0;
+	int			argc = 0, n;
 	pid_t			pid;
 	struct pid_e		*p;
 
@@ -9285,14 +9284,16 @@ setautorun(const char *selector, const char *value, int flags)
 	if (getenv("SWM_STARTED"))
 		return (0);
 
-	bzero(s, sizeof s);
-	if (sscanf(value, "ws[%d]:%1023c", &ws_id, s) != 2)
+	n = 0;
+	if (sscanf(value, "ws[%d]:%n", &ws_id, &n) != 1 || n == 0 ||
+	    value[n] == '\0')
 		errx(1, "invalid autorun entry, should be 'ws[<idx>]:command'");
+	value += n;
 	ws_id--;
 	if (ws_id < 0 || ws_id >= workspace_limit)
 		errx(1, "autorun: invalid workspace %d", ws_id + 1);
 
-	sp = str = expand_tilde((char *)&s);
+	sp = str = expand_tilde(value);
 
 	/*
 	 * This is a little intricate
@@ -9343,9 +9344,8 @@ int
 setlayout(const char *selector, const char *value, int flags)
 {
 	struct workspace	*ws;
-	int			ws_id, i, x, mg, ma, si, ar;
+	int			ws_id, i, x, mg, ma, si, ar, n;
 	int			st = SWM_V_STACK, num_screens;
-	char			s[1024];
 	bool			f = false;
 
 	/* suppress unused warnings since vars are needed */
@@ -9355,27 +9355,28 @@ setlayout(const char *selector, const char *value, int flags)
 	if (getenv("SWM_STARTED"))
 		return (0);
 
-	bzero(s, sizeof s);
-	if (sscanf(value, "ws[%d]:%d:%d:%d:%d:%1023c",
-	    &ws_id, &mg, &ma, &si, &ar, s) != 6)
+	n = 0;
+	if (sscanf(value, "ws[%d]:%d:%d:%d:%d:%n",
+	    &ws_id, &mg, &ma, &si, &ar, &n) != 5 || n == 0 || value[n] == '\0')
 		errx(1, "invalid layout entry, should be 'ws[<idx>]:"
 		    "<master_grow>:<master_add>:<stack_inc>:<always_raise>:"
 		    "<type>'");
+	value += n;
 	ws_id--;
 	if (ws_id < 0 || ws_id >= workspace_limit)
 		errx(1, "layout: invalid workspace %d", ws_id + 1);
 
-	if (strcasecmp(s, "vertical") == 0)
+	if (strcasecmp(value, "vertical") == 0)
 		st = SWM_V_STACK;
-	else if (strcasecmp(s, "vertical_flip") == 0) {
+	else if (strcasecmp(value, "vertical_flip") == 0) {
 		st = SWM_V_STACK;
 		f = true;
-	} else if (strcasecmp(s, "horizontal") == 0)
+	} else if (strcasecmp(value, "horizontal") == 0)
 		st = SWM_H_STACK;
-	else if (strcasecmp(s, "horizontal_flip") == 0) {
+	else if (strcasecmp(value, "horizontal_flip") == 0) {
 		st = SWM_H_STACK;
 		f = true;
-	} else if (strcasecmp(s, "fullscreen") == 0)
+	} else if (strcasecmp(value, "fullscreen") == 0)
 		st = SWM_MAX_STACK;
 	else
 		errx(1, "invalid layout entry, should be 'ws[<idx>]:"