Change things around so that proper quoting of values interpolated into
the BKI data by initdb is the responsibility of initdb, not something
we half-heartedly handle by putting double quotes into the raw BKI data.
(Note: experimentation shows that it still doesn't work to put a double
quote into the initial superuser username, but that's the fault of
inadequate quoting while interpolating the name into SQL scripts;
the BKI aspect of it works fine now.)

Having done that, we can remove the special-case handling of values
that look like "something" from genbki.pl, and instead teach it to
escape double --- and single --- quotes properly.  This removes the
nowhere-documented need to treat those specially in the BKI source
data; whatever you write will be passed through unchanged into the
inserted data value, modulo Perl's rules about single-quoted strings.

Add documentation explaining the (pre-existing) handling of backslashes
in the BKI data.

Per an earlier discussion with John Naylor.

Discussion: https://postgr.es/m/CAJVSVGUNao=-Q2-vAN3PYcdF5tnL5JAHwGwzZGuYHtq+Mk_9ng@mail.gmail.com