Bug in file access functions

The attached patch fixes a couple of minor issues in the file access
functions added for 8.1. These seem to have been introduced in recent
changes following the original patch.

1) Stop them rejecting paths that use Windows backslash directory
seperators.

2) Allow absolute paths to files into the data dir, as we already do for
the log dir. This allows queries like the following to work:

select pg_stat_file((select setting from pg_settings where name =
'config_file'));

Without, a relative path must be constructed which is obviously
non-trivial in SQL.

Please apply for beta 2.

Regards, Dave