Allow a -D'date' option to be included with a -r'branch'

Allow a -D'date' option to be included with a -r'branch'
on a checkout.
this allows us to do:
cd /usr/src/sys
cvs update -rRELENGE_2_2 -D"Yesterday"
which has been a feature sorely needed for any project with active branches.

warning: this breaks on usr.sbin/pkg_install for some reason.
everything else works as advertised.
(other things allready break on pkg_install, so it's not the fault of
this patch, it just falls faul of another bug somewhere)
If I had more time I'd make -r always accept the same syntax as -j (tag:data)