rm: improve efficiency of rm -r (without -f) from O(N^2) to O(N)

rm: improve efficiency of rm -r (without -f) from O(N^2) to O(N)

where N is the depth of the deepest hierarchy rm is processing.
* src/remove.c (write_protected_non_symlink): Use faccessat to
avoid O(N)-per-entry cost of calling euidaccess.
* m4/jm-macros.m4 (coreutils_MACROS): Check for faccessat.