add posix_close, accepted for inclusion in the next issue of POSIX

this is purely a wrapper for close since Linux does not support EINTR
semantics for the close syscall.

src/unistd/posix_close.c

@@ -0,0 +1,6 @@
+#include <unistd.h>
+
+int posix_close(int fd, int flags)
+{
+	return close(fd);
+}