MFC r316487, r316516, r316546, r316567: One big nop

r316487:
Allow command modifiers (fast, quiet etc.) to be stacked in any order.
Add a "debug" modifier that sets rc_debug.

r316516:
r316487 altered the defined values of rc_force from "yes" (for yes)
and NULL (for no) to "no" (for no) and no change to the definition
of yes. Two rc.d scripts, dhclient and bgfsck check rc_force for
yesi, using test -n, and no, using test -z. The redefinition of
yes and no by r316487 caused rc.d/dhclient, when invoked by devd
using a devd.conf rule, to assign DHCP assigned IP addresses for
interfaces with statically assigned interfaces, breaking boot.
Point of breakage was at line 25 of etc/rc.d/dhclient (r301068)
where $rc_force needs to be NULL.

X-MFC with: r316487

r316546:
Revert r316516. des@ asked that r316516 be reverted so that he can spend
a little more time getting r316487 right.

Requested by: des@

r316567:
Revert r316487. It is broken, causing boot to fail due to line 25 in
etc/rc.d/dhclient unconditionally testing true when called by a devd
rule during boot, ignoring statically assigned IP addresses in rc.conf.

Requested by: des@