exportfs: Use xlog() for error reporting

exportfs already invokes xlog_open() because libexport.a uses xlog()
exclusively for error reporting and debugging messages.  If we can
use xlog() throughout exportfs itself, that enables xlog debugging
messages everywhere in the code path.

In addition, use xlog() instead of fprintf(stderr) for reporting
errors in exportfs.c, to be consistent with libexport.a and other
components of nfs-utils.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>