MSVC: Substitute $(top_builddir) in REGRESS_OPTS.

Commit d7cdf6ee36adeac9233678fb8f2a112e6678a770 introduced a usage
thereof.  Back-patch to 9.0, like that commit.
Report success when Windows kill() emulation signals an exiting process.

This is consistent with the POSIX verdict that kill() shall not report
ESRCH for a zombie process.  Back-patch to 9.0 (all supported versions).
Test code from commit d7cdf6ee36adeac9233678fb8f2a112e6678a770 depends
on it, and log messages about kill() reporting "Invalid argument" will
cease to appear for this not-unexpected condition.
Rearrange documentation paragraph describing pg_relation_size().

Break the list of available options into an <itemizedlist> instead of
inline sentences.  This is mostly motivated by wanting to ensure that the
cross-references to the FSM and VM docs don't cross page boundaries in PDF
format; but it seems to me to read more easily this way anyway.  I took the
liberty of editorializing a bit further while at it.

Per complaint from Magnus about 9.0.18 docs not building in A4 format.
Patch all active branches so we don't get blind-sided by this particular
issue again in future.