Add and remove talkgroups via DTMF

[dh0dm]

This patch is intended more for hotspots than repeaters.
I'd like the ability to "(un)mute" some static talkgroups for any time for example due to traffic - but I like to be standby on my other tgs etc.
After a restart of svxlink all my static tgs are active again

There are 2 features implemented:

1. Remove any talkgroup (not just the temp ones) from monitor via DTMF.
2. A second timeout setting for temp. monitored talkgroups. The idea is to take a TG back in forever until removed. Thats why the default timeout is 10 years ;-). It could be used for shorter periods as well (few hours), a config variable is provided.

[sm0svx]

This PR need some more work.

I don't like that three commands are used for one feature. I'd like to collect all functionality under command 4. Suggesting that the "operation" is optionally added after the talk group separated by a star. Assuming a command prefix of "9" consider the following.

• 941234 -- Just as now, add TG#1234 for temporary monitoring with default timeout
• 941234*24 -- Add TG#1234 for temporary monitoring with timeout 24h.
• 941234*0 -- Remove TG#1234 from monitoring

It should be possible to configure the node to allow or disallow removal of static talk groups. Most node sysops probably don't want people randomly removing preconfigured talk groups.

It should be possible to configure the maximum time for temporarily monitoring a talk group.