More recent firmware appears to have issues even after a clean shutdown,
make sure to fully reset the DSP before starting the BTS software.

... and warn if the size is not what we expect.

This is required to work with sysmobts-v2 firmware >= v2.1, as the
SuperFemto_Prim_t is now larger than the GsmL1_Prim_t.

We need to have a documentation for each possible value, add some simple
ones for the available clock sources.

Make the clock names lower case to match with the vty command.

Make this code a bit easier to read by moving the payload setting
into a new method.

SI5/SI6 and other messages on SACCH need the C/R and the EA bit set in
the LAPDm header.  Most devices accept a broken header, but especially
the Wavecom Q2686 responds with tons of RR STATUS messages if there is
any invalid bit.