MFC r324672:
ifnet(9): split ifc_alloc_unit() (should simplify code flow)

Allocate smallest unit number from pool via ifc_alloc_unit_next()
and exact unit number (if available) via ifc_alloc_unit_specific().

While here, address possible deadlock (mentioned in PR).

PR: 217401
Differential Revision: https://reviews.freebsd.org/D12551

MFC r279538:

Fix group membership of cloned interfaces when one is moved by
if_vmove().

In if_vmove(), if_detach_internal() and if_attach_internal() were
called in series to detach and reattach the interface. When
detaching, if_delgroup() was called and the interface leaves all of
the group membership. And then upon attachment, if_addgroup(ifp,
IFG_ALL) was called and it joined only "all" group again.

This had a problem. Normally, a cloned interface automatically joins
a group whose name is ifc_name of the cloner in addition to "all"
upon creation. However, if_vmove() removed the membership and did
not restore upon attachment.

Approved by: re (gjb)

MFC r279538:

Fix group membership of cloned interfaces when one is moved by
if_vmove().

In if_vmove(), if_detach_internal() and if_attach_internal() were
called in series to detach and reattach the interface. When
detaching, if_delgroup() was called and the interface leaves all of
the group membership. And then upon attachment, if_addgroup(ifp,
IFG_ALL) was called and it joined only "all" group again.

This had a problem. Normally, a cloned interface automatically joins
a group whose name is ifc_name of the cloner in addition to "all"
upon creation. However, if_vmove() removed the membership and did
not restore upon attachment.

Approved by: re (gjb)