Allow records to span multiple lines in pg_hba.conf and pg_ident.conf.

Allow records to span multiple lines in pg_hba.conf and pg_ident.conf.

A backslash at the end of a line now causes the next line to be appended
to the current one (effectively, the backslash and newline are discarded).
This allows long HBA entries to be created without legibility problems.

While we're here, get rid of the former hard-wired length limit on
pg_hba.conf lines, by using an expansible StringInfo buffer instead
of a fixed-size local variable.

Since the same code is used to read the ident map file, these changes
apply there as well.

Fabien Coelho, reviewed by Justin Pryzby and David Zhang

Discussion: https://postgr.es/m/alpine.DEB.2.21.2003251906140.15243@pseudo