libc - Adopt more linux-friendly basename() and dirname()

libc - Adopt more linux-friendly basename() and dirname()

* Essentially take the FreeBSD basename() and dirname() code, changing
  the API to be more linux-friendly.  This reduces work required in dports.

* Adjust numerous cases in our base code that assumed the old prototypes
  and side effects.

* The passed-in string pointers are no longer const and can be modified
  in-place by the functions.  The returned value is no longer sometimes
  dynamically allocated and will instead either return a constant string,
  a portion of the passed in string, or a modified portion of the
  passed-in string.

  The API unfortunately has to be declared to return a char * instead of
  a const char *, even though it sometimes returns pointers to constant
  strings.

  The API is now thread-safe.

* The API is still a huge historical mess but at least the ports for
  the most part assume the above behavior now due to linux doing the
  same thing.

Reviewed-by: multiple people