uip/timer.h: Remove semicolon and add brackets to timer.h macro definitions

This commit fixes extraneous semicolon and missing brackets in timer.h macro
definitions.
src/timer.h: Added timer_tryreset and timer_tryrestart

These macro definitions can be used to replace constructs like this:

if (timer_expired(&timer)) {
	timer_restart(&timer);
	...
}

With:

if (timer_tryrestart(&timer)) {
	...
}