pgsql: Cosmetic improvements for default text search parser's ts_headli

Cosmetic improvements for default text search parser's ts_headline code.

This code was woefully unreadable and under-commented. Try to improve
matters by adding comments, using some macros to make complicated
if-tests more readable, using boolean type where appropriate, etc.
There are a couple of tiny coding improvements too, but this commit
includes (I hope) no behavioral change.

Nonetheless, back-patch as far as 9.6, because a followup bug-fixing
commit depends on this.

Discussion: https://postgr.es/m/16345-2e0cf5cddbdcd3b4@postgresql.org

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/afab3998685f61d07c3ae05bf22fc5f2a0e92557

Modified Files
--------------
src/backend/tsearch/wparser_def.c | 273 ++++++++++++++++++++++++--------------
1 file changed, 176 insertions(+), 97 deletions(-)