wrapper: use uname(3) to find OS version

We warn if the OS is not "Darwin".

Major versions prior to 10.2 are not fully decoded (we have no
"attach" implementation for 10.4 and earlier anyway).

TODO

@@ -1,4 +1,3 @@
-wrapper: detect OS version: probably uname(3): warn non-Darwin
 wrapper: switch 10.6 to detach "detach from console" from 10.6's screen patch?
     We want to move to newer "detach from console" function just in
     case the "move subset to user" one is removed in a future

user-session-wrapper.c

@@ -1,11 +1,12 @@
 #include <string.h>    /* strlen, strcpy, strerror, strcmp, strrchr */
 #include <stdarg.h>    /* va_...   */
 #include <stdio.h>     /* fprintf, vfprintf  */
-#include <stdlib.h>    /* malloc, exit, free */
+#include <stdlib.h>    /* malloc, exit, free, atoi */
 #include <dlfcn.h>     /* dlsym    */
 #include <stdint.h>    /* uint64_t */
 #include <unistd.h>    /* execvp   */
 #include <sys/errno.h> /* errno    */
+#include <sys/utsname.h> /* uname  */
 
 #if 0
 void * _vprocmgr_move_subset_to_user(uid_t target_user, const char *session_type, uint64_t flags); /* 10.6 */
@@ -29,12 +30,40 @@ void die(int ev, const char *fmt, ...) {
     exit(ev);
 }
 
+/*
+ * ##__VA_ARGS_ is a GNU CPP extension to invoke with zero extra args.
+ * Expects f to be a literal string (for compile-time concatenation).
+ */
+#define die_errno(x,f,...) die((x),(f ": %s"),##__VA_ARGS__,strerror(errno))
+
 int main(int argc, char *argv[]) {
     if (argc < 2)
         die(1, "usage: %s <program> [args...]", argv[0]);
 
-    /* TODO: detect OS version */
-    unsigned int os = 1060;
+    unsigned int os = 0;
+
+    struct utsname u;
+    if (uname(&u))
+        die_errno(2, "uname failed");
+    if (strcmp(u.sysname, "Darwin"))
+        fprintf(stderr, "warning: unsupported OS sysname: %s\n", u.sysname);
+
+    char *rest, *whole = strdup(u.release);
+    if (!whole)
+        die_errno(2, "strdup failed");
+    rest = whole;
+    strsep(&rest, ".");
+    if (whole && *whole && whole != rest) {
+        int major = atoi(whole);
+        if (major >= 6) /* 10.2 -- 10.7 */
+            os = 1000 + (major-4) * 10;
+        else /* 10.1, 10.0 and prior betas/previews */
+            os = 1000;
+    }
+    else
+        fprintf(stderr, "warning: unparsable major release number: '%s'\n", u.release);
+
+    free(whole);
 
     if (os < 1050) {
         fprintf(stderr, "warning: unsupported old OS, trying as if it were 10.5\n");