xspawn: Check X_OK even without $PATH resolution

Not all posix_spawn() implementations use errno to report execv()
failures from the child process, as that requires either a kernel
posix_spawn() implementation or a pipe to pass the error back.

This should fix tests/posix/exec_nonexistent on OpenBSD and HPPA.

Link: https://buildd.debian.org/status/fetch.php?pkg=bfs&arch=hppa&ver=3.3.1-1&stamp=1717489148&raw=0

src/xspawn.c

@@ -426,8 +426,17 @@ static int bfs_resolve_early(struct bfs_resolver *res, const char *exe, const st
 	};
 
 	if (bfs_can_skip_resolve(res, ctx)) {
-		res->done = true;
-		return 0;
+		// Do this check eagerly, even though posix_spawn()/execv() also
+		// would, because:
+		//
+		//     - faccessat() is faster than fork()/clone() + execv()
+		//     - posix_spawn() is not guaranteed to report ENOENT
+		if (xfaccessat(AT_FDCWD, exe, X_OK) == 0) {
+			res->done = true;
+			return 0;
+		} else {
+			return -1;
+		}
 	}
 
 	res->path = getenv("PATH");

tests/xspawn.c

@@ -179,6 +179,8 @@ static bool check_resolve(void) {
 
 	ret &= bfs_echeck(!bfs_spawn_resolve("eW6f5RM9Qi") && errno == ENOENT);
 
+	ret &= bfs_echeck(!bfs_spawn_resolve("bin/eW6f5RM9Qi") && errno == ENOENT);
+
 	return ret;
 }