Add file_extend_method=posix_fallocate,write_zeros.

Add file_extend_method=posix_fallocate,write_zeros.

Provide a way to disable the use of posix_fallocate() for relation
files.  It was introduced by commit 4d330a61bb1.  The new setting
file_extend_method=write_zeros can be used as a workaround for problems
reported from the field:

 * BTRFS compression is disabled by the use of posix_fallocate()
 * XFS could produce spurious ENOSPC errors in some Linux kernel
   versions, though that problem is reported to have been fixed

The default is file_extend_method=posix_fallocate if available, as
before.  The write_zeros option is similar to PostgreSQL < 16, except
that now it's multi-block.

Backpatch-through: 16
Reviewed-by: Jakub Wartak <jakub.wartak@enterprisedb.com>
Reported-by: Dimitrios Apostolou <jimis@gmx.net>
Discussion: https://postgr.es/m/b1843124-fd22-e279-a31f-252dffb6fbf2%40gmx.net