Revert "Execute pkg scripts via posix_spawn(3)."

This reverts commit e33f6354c3.

@Gottox pointed out why this won't work.

lib/external/fexec.c

@@ -37,7 +37,6 @@
 #include <stdarg.h>
 #include <stdlib.h>
 #include <string.h>
-#include <spawn.h>
 
 #undef _DEFAULT_SOURCE
 #undef _BSD_SOURCE
@@ -49,10 +48,7 @@ pfcexec(struct xbps_handle *xhp, const char *file, const char **argv)
 	pid_t child;
 	int status;
 
-	if (posix_spawn(&child, file, NULL, NULL,
-			(char* const*)__UNCONST(argv), NULL) != 0)
-		return -1;
-
+	child = vfork();
 	switch (child) {
 	case 0:
 		/*