util: make util_get_process_exec_path work on FreeBSD w/o procfs

sysctl is the correct way of getting the current executable's path.
procfs is not mounted by default.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1598>
(cherry picked from commit 98dbd01a96)

.pick_status.json

@@ -1093,7 +1093,7 @@
         "description": "util: make util_get_process_exec_path work on FreeBSD w/o procfs",
         "nominated": true,
         "nomination_type": 0,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null
     },

src/util/u_process.c

@@ -44,6 +44,11 @@
 #include <mach-o/dyld.h>
 #endif
 
+#if DETECT_OS_FREEBSD
+#include <sys/types.h>
+#include <sys/sysctl.h>
+#endif
+
 #if defined(__linux__) && defined(HAVE_PROGRAM_INVOCATION_NAME)
 
 static char *path = NULL;
@@ -211,6 +216,13 @@ util_get_process_exec_path(char* process_path, size_t len)
    int result = _NSGetExecutablePath(process_path, &bufSize);
 
    return (result == 0) ? strlen(process_path) : 0;
+#elif DETECT_OS_FREEBSD
+   int mib[4] = { CTL_KERN, KERN_PROC, KERN_PROC_PATHNAME, -1 };
+
+   (void) sysctl(mib, 4, process_path, &len, NULL, 0);
+   process_path[len - 1] = '\0';
+
+   return len;
 #elif DETECT_OS_UNIX
    ssize_t r;