portability: fix problems compiling with C++ compiler

Michael McConville suggested removing a "redundant" cast of
the return value from malloc(), based on a change in the
OpenBSD version of unifdef. This is a good suggestion for
a purely C program, but the cast is required in C++.

Carsten Hey contributed some portability improvements in
2012 which included this cast, so that unifdef can be
compiled with a C++ compiler. Unfortunately since then
there have been several regressions in C++ support.

This commit fixes those regressions.

unifdef.c

@@ -1626,7 +1626,7 @@ xstrdup(const char *start, const char *end)
 
 	if (end < start) abort(); /* bug */
 	n = (size_t)(end - start) + 1;
-	s = malloc(n);
+	s = (char *)malloc(n);
 	if (s == NULL)
 		err(2, "malloc");
 	snprintf(s, n, "%s", start);

win32/unifdef.h

@@ -59,7 +59,7 @@
 #define snprintf c99_snprintf
 
 /* win32.c */
-int replace(const char *old, const char *new);
+int replace(const char *oldname, const char *newname);
 FILE *mktempmode(char *tmp, int mode);
 FILE *fbinmode(FILE *fp);
 int c99_snprintf(char *buf, size_t buflen, const char *format, ...);

win32/win32.c

@@ -31,11 +31,11 @@
  * remove anything that might be in the way before renaming.
  */
 int
-replace(const char *old, const char *new)
+replace(const char *oldname, const char *newname)
 {
-	if (remove(new) < 0 && errno != ENOENT)
-		warn("can't remove \"%s\"", new);
-	return (rename(old, new));
+	if (remove(newname) < 0 && errno != ENOENT)
+		warn("can't remove \"%s\"", newname);
+	return (rename(oldname, newname));
 }
 
 FILE *
@@ -77,7 +77,7 @@ int c99_snprintf(char *buf, size_t buflen, const char *format, ...)
 	/* Paranoia about off-by-one errors in _snprintf() */
 	tmplen = outlen + 2;
 
-	tmp = malloc(tmplen);
+	tmp = (char *)malloc(tmplen);
 	if (tmp == NULL)
 		err(2, "malloc");
 	va_start(ap, format);