PL/Python: Fix type mixup

Memory was allocated based on the sizeof a type that was not the type of
the pointer that the result was being assigned to.  The types happen to
be of the same size, but it's still wrong.
Refactor checksumming code to make it easier to use externally.

pg_filedump and other external utility programs are likely to want to be
able to check Postgres page checksums.  To avoid messy duplication of code,
move the checksumming functionality into an exported header file, much as
we did awhile back for the CRC code.

In passing, get rid of an unportable assumption that a static char[] array
will be word-aligned, and do some other minor code beautification.
Remove special-case treatment of LOG severity level in standalone mode.

elog.c has historically treated LOG messages as low-priority during
bootstrap and standalone operation.  This has led to confusion and even
masked a bug, because the normal expectation of code authors is that
elog(LOG) will put something into the postmaster log, and that wasn't
happening during initdb.  So get rid of the special-case rule and make
the priority order the same as it is in normal operation.  To keep from
cluttering initdb's output and the behavior of a standalone backend,
tweak the severity level of three messages routinely issued by xlog.c
during startup and shutdown so that they won't appear in these cases.
Per my proposal back in December.