registrar: move cmdutil.command to registrar module (API)
cmdutil.command wasn't a member of the registrar framework only for a
historical reason. Let's make that happen. This patch keeps cmdutil.command
as an alias for extension compatibility.
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2