login: make hold-off timeout configurable

login: make hold-off timeout configurable

This introduces 'HoldoffTimeoutSec' to logind.conf to make
IGNORE_LID_SWITCH_{SUSPEND,STARTUP}_USEC configurable.

Background: If an external monitor is connected, or if the system is
docked, we want to ignore LID events. This is required to support setups
where a laptop is used with external peripherals while the LID is closed.
However, this requires us to probe all hot-plugged devices before reacting
to LID events. But with modern buses like USB, the standards do not impose
any timeout on the slots, so we have no chance to know whether a given
slot is used or not. Hence, after resume and startup, we have to wait a
fixed timeout to give the kernel a chance to probe devices. Our timeout
has always been generous enough to support even the slowest devices.
However, a lot of people didn't use these features and wanted to disable
the hold-off timer. Now we provide a knob to do that.