Code review of recycling WAL segments in a restartpoint.

Seems cleaner to get the currently-replayed TLI in the same call to
GetXLogReplayRecPtr that we get the WAL position. Make it more clear in the
comment what the code does when recovery has already ended
(RecoveryInProgress() will set ThisTimeLineID in that case). Finally, make
resetting ThisTimeLineID afterwards more explicit.
Additional spelling corrections

A few more minor spelling corrections, no functional changes.

Thom Brown
Put back allow_system_table_mods check in heap_create().

This reverts commit a475c6036752c26dca538632b68fd2cc592976b7.

Erik Rijkers reported back in January 2013 that after the patch, if you do
"pg_dump -t myschema.mytable" to dump a single table, and restore that in
a database where myschema does not exist, the table is silently created in
pg_catalog instead. That is because pg_dump uses
"SET search_path=myschema, pg_catalog" to set schema the table is created
in. While allow_system_table_mods is not a very elegant solution to this,
we can't leave it as it is, so for now, revert it back to the way it was
previously.
Add semicolons to eval'd strings to hide a minor Perl behavioral change.

"eval q{foo}" used to complain that the error was on line 2 of the eval'd
string, because eval internally tacked on "\n;" so that the end of the
erroneous command was indeed on line 2.  But as of Perl 5.18 it more
sanely says that the error is on line 1.  To avoid Perl-version-dependent
regression test results, use "eval q{foo;}" instead in the two places
where this matters.  Per buildfarm.

Since people might try to use newer Perl versions with older PG releases,
back-patch as far as 9.0 where these test cases were added.
Fix off-by-one in pg_xlogdump -r option.

Because of the bug, -r would not accept the rmgr with the highest ID.