tlp-functions: add get_sys_power_supply()

A variant of get_power_state() that returns 2 ("unknown") rather
than 1 ("battery") when neither a battery nor an online AC power source
could be found.

get_power_state() wraps this function and behaves as before
(AC->0, anything else->1).

Signed-off-by: André Erdmann <dywi@mailerd.de>
tlp-stat: report "unknown" power source

Ref: github issue #109
Signed-off-by: André Erdmann <dywi@mailerd.de>