Use "WAL segment" instead of "log segment" consistently in user-facing messages

Hi,

It looks like we use "log segment" in various user-facing messages.
The term "log" can mean server logs as well. The "WAL segment" suits
well here and it is consistently used across the other user-facing
messages [1].

Here's a small patch attempting to consistently use the "WAL segment".

Thoughts?

[1]
pg_log_error("could not fetch WAL segment size: got %d rows and %d
fields, expected %d rows and %d or more fields",
pg_log_error("WAL segment size could not be parsed");
pg_log_error(ngettext("WAL segment size must be a power of two between
1 MB and 1 GB, but the remote server reported a value of %d byte",
printf(_("WARNING: invalid WAL segment size\n"));
printf(_("Bytes per WAL segment:                %u\n"),
fatal_error(ngettext("WAL segment size must be a power of two between
1 MB and 1 GB, but the WAL file \"%s\" header specifies %d byte",
errmsg("requested WAL segment %s has already been removed",
elog(DEBUG2, "removed temporary WAL segment \"%s\"", path);

Regards,
Bharath Rupireddy.