pipe_read_line for reading arbitrary strings

When skimming through pg_rewind during a small review I noticed the use of
pipe_read_line for reading arbitrary data from a pipe, the mechanics of which
seemed odd.

Commit 5b2f4afffe6 refactored find_other_exec() and broke out pipe_read_line()
as a static convenience routine for reading a single line of output to catch a
version number.  Many years later, commit a7e8ece41 exposed it externally in
order to read a GUC from postgresql.conf using "postgres -C ..".  f06b1c598
also make use of it for reading a version string much like find_other_exec().
Funnily enough, while now used for arbitrary string reading the variable is
still "pgver".

Since the function requires passing a buffer/size, and at most size - 1 bytes
will be read via fgets(), there is a truncation risk when using this for
reading GUCs (like how pg_rewind does, though the risk there is slim to none).

If we are going to continue using this for reading $stuff from pipes, maybe we
should think about presenting a nicer API which removes that risk?  Returning
an allocated buffer which contains all the output along the lines of the recent
pg_get_line work seems a lot nicer and safer IMO.

The attached POC diff replace fgets() with pg_get_line(), which may not be an
Ok way to cross the streams (it's clearly not a great fit), but as a POC it
provided a neater interface for reading one-off lines from a pipe IMO.  Does
anyone else think this is worth fixing before too many callsites use it, or is
this another case of my fear of silent subtle truncation bugs?  =)

--
Daniel Gustafsson