initial implementation of posix_spawn

file actions are not yet implemented, but everything else should be
mostly complete and roughly correct.

src/process/posix_spawnp.c

@@ -0,0 +1,14 @@
+#include <spawn.h>
+#include <unistd.h>
+
+int __posix_spawnx(pid_t *, const char *,
+	int (*)(const char *, char *const *),
+	const posix_spawn_file_actions_t *,
+	const posix_spawnattr_t *, char **, char **);
+
+int posix_spawnp(pid_t *res, const char *file,
+	const posix_spawn_file_actions_t *fa,
+	const posix_spawnattr_t *attr, char **argv, char **envp)
+{
+	return __posix_spawnx(res, file, execvp, fa, attr, argv, envp);
+}