Added support for MultiMobile MT9234MU modem and a few other small changes

[foureight84]

I'm not sure if this repository is still being maintained since Bruce's passing. It's an incredible project and I just recently stumbled across it a few weeks ago.

I've been using it with a Conexant CX93010-based modem for a bit but I have found the voicemail quality to be fairly noisy (at least on the generic model I bought from Amazon). I picked up a cheap MultiMobile MT9234MU on eBay that seems to have much higher recording quality with the same 8-bit PCM compression.

I've added support for this modem and created a pull request for the changes.
Other changes include:

• Added REC_VM_MAX_DURATION and SERIAL_BAUD_RATE to configuration. SERIAL_BAUD_RATE for MT9234MU should be 115200 for the best audio quality from testing
• Added dial tone detection to message recording
• Disabled internal speaker on modem init. MT9234MU has a built-in speaker
• Bumped version to 1.1.2

In case someone wants to give it a go, you can find changes on my forked repo:

• master branch for use with a raspberry pi
• no-gpio branch for use on non-raspberry pi machines
• Docker configuration repo