Commands to manage secret are moved from virsh.c to virsh-secret.c,
with a few helpers for secret command use.
* virsh.c: Remove secret commands and a few helpers.
(vshCommandOptSecret, and vshCommandOptSecretBy)
* virsh-secret.c: New file, filled with secret commands and its helpers.
* po/POTFILES.in: Add virsh-secret.c
* cfg.mk: Skip to check config.h including for virsh-secret.c
