Shorten callsigns when possible

To keep radio communications brief, shorten callsigns whenever possible, according to the following rules:

• Drop the common section of the callsign of all stations on the net (ALPHA 1-1 ←→ ALPHA 1-2 becomes (1 ←→ 2)
• Drop the numerals when only a single station of that callsign is on the net (ALPHA 1 ←→ BRAVO 1 becomes ALPHA ←→ BRAVO)