NEWS for v5.1.0

This is primarily a maintenance release.

The most notable update is that regular expression (RE) searches are
supported again.  In ancient times, most supported RE searches, but
was RE support was dropped was dropped when UTF-8 support was added.
The default is still to use a non-RE search. The -r command line
switch may be used to make the default a RE-based search.  Within
most, ":r" may be used to toggle the search type between RE and
non-RE.