Allow NULL-value comments, and form "KEY="
Allow NULL-value comments, and form "KEY="

This commit changes the policy of fs_comment_validate_byname() to
allow NULL-value comments, and changes fs_comment_new() to handle
the case where value=NULL, and where the comment has no value but
has an = sign.

This commit should pass "make check", including the policy change
introduced in the previous commit.

kfish/liboggz,brion/liboggz,kfish/liboggz,brion/liboggz,brion/liboggz,kfish/liboggz,brion/liboggz