Be more wary of false matches in initdb's replace_token().

Be more wary of false matches in initdb's replace_token().

Do not replace the target string unless the occurrence is surrounded
by whitespace or line start/end.  This avoids potential false match
to a substring of a field.  While we've not had trouble with that
up to now, the next patch creates hazards of false matches to
POSTGRES within an ACL field.

There is one call site that needs adjustment, as it was presuming
it could write "::1" and have that match "::1/128".  For all the
others, this restriction is okay and strictly safer.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/183292bb-4891-4c96-a3ca-e78b5e0e1358@dunslane.net