Move pattern selectivity code from selfuncs.c to like_support.c.

Move pattern selectivity code from selfuncs.c to like_support.c.

While at it, refactor patternsel() a bit so that it can be used from
the LIKE/regex planner support functions as well.  This makes the
planner able to deal equally well with either operator or function
syntax for these operations.  I'm not excited about that as a feature
in itself, but it provides a nice model for extensions to follow if
they want such behavior for their operations.

This change localizes the use of pattern_fixed_prefix() and
make_greater_string() so that they no longer need be exported.
(We might get pushback from extensions about that, perhaps,
in which case I'd be inclined to re-export them in a new header
file like_support.h.)

This reduces the bulk of selfuncs.c a fair amount, removing ~1370
lines or about one-sixth of that file; it's still too big, but this
is progress.

Discussion: https://postgr.es/m/24537.1550093915@sss.pgh.pa.us