os-posix: Provide new -runas <uid>:<gid> facility

os-posix: Provide new -runas <uid>:<gid> facility

This allows the caller to specify a uid and gid to use, even if there
is no corresponding password entry.  This will be useful in certain
Xen configurations.

We don't support just -runas <uid> because: (i) deprivileging without
calling setgroups would be ineffective (ii) given only a uid we don't
know what gid we ought to use (since uids may eppear in multiple
passwd file entries with different gids).

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
CC: Daniel P. Berrange <berrange@redhat.com>
CC: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Markus Armbruster <armbru@redhat.com>