tweak begpar() to rely on fileage instead of NULL for its check to

tweak begpar() to rely on fileage instead of NULL for its check to
detect the top of the file, and make it return NULL if foo is NULL, just
as inpar() does

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3126 35c25a1d-7b9e-4130-9fde-d3aeb78583b8