Rework internal command generation of pg_rewind

Rework internal command generation of pg_rewind

pg_rewind generates and executes internally up to two commands to work
on the target cluster, depending on the options given by its caller:
- postgres -C to retrieve the value of restore_command, when using
-c/--restore-target-wal.
- postgres --single to enforce recovery once and get the target cluster
in a clean shutdown state.

Both commands have been applying incorrect quoting rules, which could
lead to failures when for example using a target data directory with
unexpected characters like CRLFs.  Those commands are now generated with
PQExpBuffer, making use of string_utils.h to quote those commands as
they should.  We may extend those commands in the future with more
options, so this makes any upcoming additions easier.

This is arguably a bug fix, but nobody has complained about the existing
code being a problem either, so no backpatch is done.

Extracted from a larger patch by the same author.

Author: Gunnar "Nick" Bluth
Discussion: https://postgr.es/m/7c59265d-ac50-b0aa-ca1e-65e8bd27642a@pro-open.de