In preparation for splitting virsh-interface.c, I found these
functions need to be declared in virsh.h, as well as one that
belongs more properly in virsh-domain.h.  Also, since we
use the VSH_BY* flags in more than one function, I improved
how they are used.

* tools/virsh.h (vshNameSorter, vshCmdHasOption): Declare.
(VSH_BYID): Turn into enum.
(vshCommandOptDomainBy): Move...
* tools/virsh-domain.h): ...here.
* tools/virsh.c: (vshNameSorter): Export.
(cmd_has_option): Rename...
(vshCmdHasOption): ...and export.
(vshCommandOptDomainBy): Move...
* tools/virsh-domain.c (vshCommandOptDomainBy): ...here, adjust
signature, and check flags.
* tools/virsh-network.c (vshCommandOptNetworkBy): Update callers.
* tools/virsh-nwfilter.c (vshCommandOptNWFilterBy): Likewise.
* tools/virsh-secret.c (vshCommandOptSecret): Likewise.
* tools/virsh-domain-monitor.c (includes): Likewise.
* tools/virsh-host.c (includes): Likewise.