Reduce memory leakage in initdb.

Reduce memory leakage in initdb.

While testing commit 3e51b278d, I noted that initdb leaks about a
megabyte worth of data due to the sloppy bookkeeping in its
string-manipulating code.  That's not a huge amount on modern machines,
but it's still kind of annoying, and it's easy to fix by recognizing
that we might as well treat these arrays of strings as
modifiable-in-place.  There's no caller that cares about preserving
the old state of the array after replace_token or replace_guc_value.

With this fix, valgrind sees only a few hundred bytes leaked during
an initdb run.

Discussion: https://postgr.es/m/2844176.1674681919@sss.pgh.pa.us