Add Whilelisting of callsigns; Enable disallow functionality

[fuelxc]

Sorry about not having tests and stuff. I have no real experience with C so I was very very hacky. I dunno if someone can help me with the tests and cleaning it up.

[fuelxc]

I will note that I have live tested it: config:

# Configuration for aprsc, an APRS-IS server for core servers

# Your unique server ID
ServerId  JEEPRSNET 

...

#
FileLimit        10000
AllowLoginCall KJ7QXU
DisallowLoginCall *

Escape character is '^]'.
# aprsc 2.1.8-gf07682dM
user KC7RZT pass 21927
# Login by user not allowed
user KJ7QXU pass 12345
# Login by unverified user not allowed
user KJ7QXU pass 22444
# logresp KJ7QXU verified, server JEEPRSNET
HP1CSL-9>PX5Q8Z,qAS,HP1UPR-1:`kKq+>/`"4o}Siguenos en @radioaficionpty_%

[snip]

This feature looks very interesting!

[hessu]

Hi, as you can see from the automatic test runs by Travis, the behaviour changes so that the existing tests fail. Please run the test suite (cd tests; make test) and fix any obvious issues. The .travis.yml in the root describes the steps to install dependencies for the tests.

In principle the feature looks good, but the PR should not break existing tests and use cases without careful consideration and good reasons to do so.