* add trau_mux implementation to relay from one incoming TRAU
channel to another one (simple voice call switching)
* add a way more generic E1 input layer, abstracting out the misdn
low-level interface. This also adds infrastructure for multiple TRX
in one BTS, as well as multiple BTS on one E1 link
* add a E1 subchannel multiplexer for sending multiple 16kbit sub-channels
one one 64kBps E1 channel
* add TRAU IDLE frame generation
* terminate bsc_hack in case there is a E1 / mISDN init error
* introduce 'e1_config.c' file with static configuration of our
E1 setup (which TRX/BTS is configured for which TEI/SAPI/E1). This should
later become a config file rather than a compiled C file.
WARNING: all this compiles but is not tested yet. Expect fix-up committs over
the next hours or so