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