entry: add the 'L' extension field, holding the matching source text.

The only (reasonable) way to get the tag's source line was to use the
xref output format.  However, this format is fixed and doesn't allow
extension fields to be specified.

A more reasonable solution is to add the extension field to the tags
file format, holding the matched source line.  This can be used by
external tags file readers, along with any other fields they may need.

import Plan 9 port from /n/9pio/contrib/anothy/src/ctags (thanks Anothy Sorace)