Clean up some copied-and-pasted code in pg_upgrade.

1. Don't reimplement S_ISDIR() and S_ISREG() badly.
2. Don't reimplement access() badly.

This code appears to have been copied from ancient versions of the
corresponding backend routines, and not patched to incorporate subsequent
fixes (see my commits of 2008-03-31 and 2010-01-14 respectively).
It might be a good idea to change it to just *call* those routines,
but for now I'll just transpose these fixes over.

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=3864afa1d1478f7a76fde1ea160a7bc8f5d5b203

Modified Files
--------------
contrib/pg_upgrade/exec.c |   69 +++-----------------------------------------
contrib/pg_upgrade/file.c |    4 +-
2 files changed, 7 insertions(+), 66 deletions(-)

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	Subject: Re: [COMMITTERS] pgsql: Reduce spurious Hot Standby conflicts from never-visible records
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