Initscripts already honor the DEVTIMEOUT variable (rh #1171917).

Don't make this a property only supported by initscripts. Every
useful property should also be supported by keyfile and it should
be accessible via D-Bus.

Also, I will soon drop NMSIfcfgConnection, so handling this would
require extra code. It's easier when DEVTIMEOUT is a regular property of
the connection profile.

The property is not yet implemented. ifcfg-rh still uses the old
implementation, and keyfile is not yet adjusted. Since both keyfile
and ifcfg-rh will both be rewritten soon, this property will be
implemented then.