single-user vs standalone in docs and messages

Some places in the docs and elog hints refer to "standalone" backends,
while the official name as used in app-postgres.html is "single-user mode",
and in fact "standalone" does not appear on that page.

This tries to standardize the other locations to use "single-user". I
think I did the right thing with the message translation files, but I can't
figure out how to test that.

I made no attempt to change code-comments, just the user-facing parts.

Cheers,

Jeff