initial implementation of posix_spawn

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

src/process/posix_spawnattr_setsigdefault.c

@@ -0,0 +1,7 @@
+#include <spawn.h>
+
+int posix_spawnattr_setsigdefault(posix_spawnattr_t *attr, const sigset_t *def)
+{
+	attr->__def = *def;
+	return 0;
+}