MFC r300775:

MFC r300775:

  Let l64a() properly null terminate its result.

  Though the buffer used by l64a() is initialized with null bytes,
  repetetive calls may end up having trailing garbage of previous
  invocations because we don't end up terminating the string.

  Instead of importing NetBSD's fix, use this opportunity to simplify this
  function dramatically, for example by just storing the Base64 character
  set in a string. There is also no need to do the bitmasking, as we can
  just use the proper integer type from <stdint.h>.

git-svn-id: svn://svn.freebsd.org/base/stable/10@302193 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f