libfrog: hoist some utilities from libxfs

libfrog: hoist some utilities from libxfs

This started with a desire to move the duplicate cmn_err declarations in
libxfs into libfrog/util.h ahead of the patch that renames libxfs_priv.h
to xfs_platform.h.

Then this patch expanded in scope when I realized that there were
several other utility functions that weren't specific to xfs; those go
in libfrog.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>