libexport.a: Allow malloc(3) failures in client_lookup() and friends

libexport.a: Allow malloc(3) failures in client_lookup() and friends

Clean up: Use malloc(3) instead of xmalloc() in client_lookup() and
client_dup(), ensuring that a failed memory allocation here doesn't
cause our process to exit suddenly.

Allocation of nfs_client records and the m_hostname string are now
consistently handled with malloc(3), calloc(3), strdup(3), and
free(3).

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