Various dhcpcd fixups.

Various dhcpcd fixups.

- fix Rebind; previous use of bind_interface was wrong
- don't use signals to effect state changes; call directly
  into the code to avoid the signal delivery and associated
  socketpair dance
- release lease and terminate if dbus goes away
- set nocarrier in dhcpcd.conf to disable carrier checking;
  we act solely on dbus msgs
- while here do some code cleanups (e.g. kill #if 0 code)

TEST=do roaming tests and observe a single dhcpcd process renew leases as required; also tested release logic and dbus drop out logic by killing dbus-daemon

[Sam Leffler, 2010-07-14 00:42:35]

This is everything we discussed except setting the rebind timeout low; I'm not
sure about doing that and in the normal case we should receive a NAK
immediately.

[Nathan Williams, 2010-07-14 14:27:09]

LGTM.