pgsql: Add a --socketdir option to pg_upgrade.

Add a --socketdir option to pg_upgrade.

This allows control of the directory in which the postmaster sockets
are created for the temporary postmasters started by pg_upgrade.
The default location remains the current working directory, which is
typically fine, but if it is deeply nested then its pathname might
be too long to be a socket name.

In passing, clean up some messiness in pg_upgrade's option handling,
particularly the confusing and undocumented way that configuration-only
datadirs were handled.  And fix check_required_directory's substantially
under-baked cleanup of directory pathnames.

Daniel Gustafsson, reviewed by Hironobu Suzuki, some code cleanup by me

Discussion: https://postgr.es/m/E72DD5C3-2268-48A5-A907-ED4B34BEC223@yesql.se

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2d34ad84303181111c6f0747186857ff50106267

Modified Files
--------------
doc/src/sgml/ref/pgupgrade.sgml | 36 ++++++++++++++++---
src/bin/pg_upgrade/option.c     | 80 ++++++++++++++++++++++-------------------
src/bin/pg_upgrade/pg_upgrade.h |  5 +--
3 files changed, 77 insertions(+), 44 deletions(-)