* Tweak isvalidatom() to treat "null" category as valid, but missing category

* Tweak isvalidatom() to treat "null" category as valid, but missing category
  as invalid (previously both where treated as invalid).

* Fix a spot inside emerge's unmerge() function that sometimes generates
  invalid atoms by adding an unnecessary "=" to the front. Even when in
  cases that resulted in a valid atom here, adding the "=" was redundant
  because dep_expand() does that automatically now (for backward compat).

svn path=/main/trunk/; revision=10516