Improvements to GetErrorContextStack()

As GetErrorContextStack() borrowed setup and tear-down code from other
places, it was less than clear that it must only be called as a
top-level entry point into the error system and can't be called by an
exception handler (unlike the rest of the error system, which is set up
to be reentrant-safe).

Being called from an exception handler is outside the charter of
GetErrorContextStack(), so add a bit more protection against it,
improve the comments addressing why we have to set up an errordata
stack for this function at all, and add a few more regression tests.

Lack of clarity pointed out by Tom Lane; all bugs are mine.
pg_upgrade:  adjust umask() calls
Since pg_upgrade -j on Windows uses threads, calling umask()
before/after opening a file via fopen_priv() is no longer possible, so
set umask() as we enter the thread-creating loop, and reset it on exit.
Also adjust internal fopen_priv() calls to just use fopen().
Backpatch to 9.3beta.
Merge branch 'fpw-compress' of github.com:MasaoFujii/postgresql into fpw-compress
Fix the bug that backup block is compressed when compress_backup_block is set to off.