find all beginning-of-line and/or end-of-line regexes once per line, not

find all beginning-of-line and/or end-of-line regexes once per line, not
just the zero-length ones; this fixes multiple replaces occurring with
them in conjunction with "*"

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@1608 35c25a1d-7b9e-4130-9fde-d3aeb78583b8