Fix patty_timer_start() to only set the running timer to the value of
the timer being started, rather than adding the value of the started
timer to the running timer
Changes:
* Implement patty_timer_expired() to determine if a timer has
expired or has gone negative
* Implement patty_timer_cancel() to set a timer to zero
* Implement patty_timer_start() to initialize a timer, add a
specified number of milliseconds, and ensure a target timer (such
as the server timer) is set for at least that value as well
* Implement patty_timer_tick() to subtract an elapsed time value
from a timer
* Remove functions specific to each timer in src/sock.c, as those
can now be handled generically by src/timer.c
* Replace calls to patty_ax25_sock_timer_t1_*() with patty_timer_*()
calls instead; reference the struct timeval objects directly
within the sock (for the time being, pending opaque structures)
