pgsql: Use our own getopt() on OpenBSD.

Use our own getopt() on OpenBSD.

Recent OpenBSD (at least 5.9 and up) has a version of getopt(3)
that will not cope with the "-:" spec we use to accept double-dash
options in postgres.c and postmaster.c.  Admittedly, that's a hack
because POSIX only requires getopt() to allow alphanumeric option
characters.  I have no desire to find another way, however, so
let's just do what we were already doing on Solaris: force use
of our own src/port/getopt.c implementation.

In passing, improve some of the comments around said implementation.

Per buildfarm and local testing.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/30197.1547835700@sss.pgh.pa.us

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/dc2dee438015ad390528c5c6f2d0118d248196f0

Modified Files
--------------
configure               |  6 +++---
configure.in            |  6 +++---
src/include/pg_getopt.h | 10 ++++++++++
src/port/getopt.c       | 11 +++++------
4 files changed, 21 insertions(+), 12 deletions(-)