xfs_db: fix metadump redirection (again)

xfs_db: fix metadump redirection (again)

In patch 4944defad4 ("xfs_db: redirect printfs when metadumping to
stdout"), we solved the problem of xfs_db printfs ending up in the
metadump stream by reassigning stdout for the duration of a stdout
metadump.  Unfortunately, musl doesn't allow stdout to be reassigned (in
their view "extern FILE *stdout" means "extern FILE * const stdout"), so
we abandon the old approach in favor of playing games with dup() to
switch the raw file descriptors.

While we're at it, fix a regression where an unconverted outf test
allows progress info to end up in the metadump stream.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>