posix_spawn: use larger stack to cover worst-case in execvpe

execvpe stack-allocates a buffer used to hold the full path
(combination of a PATH entry and the program name)
while searching through $PATH, so at least
NAME_MAX+PATH_MAX is needed.

The stack size can be made conditionally smaller
(the current 1024 appears appropriate)
should this larger size be burdensome in those situations.

src/process/posix_spawn.c

@@ -152,7 +152,7 @@ int __posix_spawnx(pid_t *restrict res, const char *restrict path,
 	char *const argv[restrict], char *const envp[restrict])
 {
 	pid_t pid;
-	char stack[1024];
+	char stack[1024+PATH_MAX];
 	int ec=0, cs;
 	struct args args;