Updated instrumentation patch

Per recent discussion, here is yet another updated version of the
instrumentation patch. Changes:

* Added guc option "disable_remote_admin", that disables any write
operations (write, unlink, rename) even for the superuser. Set as
PGC_POSTMASTER so it cannot be changed remotely.
I put this under "file locations", because that's where all the other
config file information is. Though that doesn't feel completely right, I
couldn't find a better place without creating a whole new category (it's
not *connection* security, after all), and if that's to be done I think
it's better if one of the committers pick name etc for it :-)

* Make sure pg_file_stat() can only be used by superuser. It lacked this
check previously.

* Updated so it applies to current cvs. This means all oids have
changed, since they were all used for other things now. Also added a
required header that had moved with the datetime stuff.

Actual code changes against the previous patch are very small.

//Magnus