Log Message:
* Make PKGSRC_TOPDIR a private variable by renaming it to _PKGSRC_TOPDIR,
  as it's only used internally by bsd.prefs.mk.

* Make _PKGSRCDIR a public variable by renaming it to PKGSRCDIR.
  Also, generate its value from ${_PKGSRC_TOPDIR} so it's less fragile
  than the old method of stripping off the last two components of
  ${.CURDIR}.  PKGSRCDIR may now be used after bsd.prefs.mk is defined.

* Change all references to _PKGSRCDIR to PKGSRCDIR.