Clarify terminology standalone backend vs. single-user mode

Most of the documentation uses "single-user mode", so use that in the
code as well.  Adjust the documentation to match the new error message
wording.  Also add a documentation index entry for "single-user mode".

Based-on-patch-by: Jeff Janes <jeff.janes@gmail.com>
doc: Fix date in EPUB manifest

If there is no <date> element, the publication date for the EPUB
manifest is taken from the copyright year.  But something like
"1996-2013" is not a legal date specification.  So the EPUB output
currently fails epubcheck.

Put in a separate <date> element with the current year.  Put it in
legal.sgml, because copyright.pl already instructs to update that
manually, so it hopefully won't be missed.
Ensure no xid gaps during Hot Standby startup

In some cases with higher numbers of subtransactions
it was possible for us to incorrectly initialize
subtrans leading to complaints of missing pages.

Bug report by Sergey Konoplev
Analysis and fix by Andres Freund
Add a comment warning against use of pg_usleep() for long sleeps.

Follow-up to commit 873ab97219caabeb2f7b390268a4fe01e2b7518c, in which
I noted that WaitLatch was a better solution in the commit log message,
but neglected to add any documentation in the code.