Improve system call documentation

This change also introduces partial faccessat() support for zipos and
makes some slight breaking changes in errno results. close() is fixed
to use `EBADF` rather than `EINVAL` and we're now using `ENOTSUP` not
`EOPNOTSUPP` to indicate that zipos doesn't support a system call yet

libc/calls/access.c

@@ -22,9 +22,14 @@
 /**
  * Checks if effective user can access path in particular ways.
  *
+ * This is equivalent to saying:
+ *
+ *     faccessat(AT_FDCWD, path, mode, 0);
+ *
  * @param path is a filename or directory
  * @param mode can be R_OK, W_OK, X_OK, F_OK
- * @return 0 if ok, or -1 and sets errno
+ * @return 0 if ok, or -1 w/ errno
+ * @see faccessat() for further documentation
  * @asyncsignalsafe
  */
 int access(const char *path, int mode) {