Fix libpq to not require user's home directory to exist.

Some people like to run libpq-using applications in environments where
there's no home directory.  We've broken that scenario before (cf commits
5b4067798 and bd58d9d88), and commit ba005f193 broke it again, by making
it a hard error if we fail to get the home directory name while looking
for ~/.pgpass.  The previous precedent is that if we can't get the home
directory name, we should just silently act as though the file we hoped
to find there doesn't exist.  Rearrange the new code to honor that.

Looking around, the service-file code added by commit 41a4e4595 had the
same disease.  Apparently, that escaped notice because it only runs when
a service name has been specified, which I guess the people who use this
scenario don't do.  Nonetheless, it's wrong too, so fix that case as well.

Add a comment about this policy to pqGetHomeDirectory, in the probably
vain hope of forestalling the same error in future.  And upgrade the
rather miserable commenting in parseServiceInfo, too.

In passing, also back off parseServiceInfo's assumption that only ENOENT
is an ignorable error from stat() when checking a service file.  We would
need to ignore at least ENOTDIR as well (cf 5b4067798), and seeing that
the far-better-tested code for ~/.pgpass treats all stat() failures alike,
I think this code ought to as well.

Per bug #14872 from Dan Watson.  Back-patch the .pgpass change to v10
where ba005f193 came in.  The service-file bugs are far older, so
back-patch the other changes to all supported branches.

Discussion: https://postgr.es/m/20171025200457.1471.34504@wrigleys.postgresql.org

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