Changes

configure.ac: lower libusb-1.0 requirement to 1.0.20

Current configure.ac requires libusb version to be >= 1.0.21.
Slackware 14.2 has libusb version 1.0.20.  Lowering the
requirement in configure.ac produces a successful build,
i.e., the actual code does not use any libusb APIs that are
new with version 1.0.21.

Change-Id: I15586757e2985184359a4488c79f2c587affeb56

e1_ts_stop: clear tx_started flag

ts->raw.tx_started flag gets set in the channelized mux process
when bytes begin arriving from the raw TS client application.
However, there is no reset of this flag anywhere, hence if a user
opens some TS, then kills that client application, then opens it
again (with the same or different client app), there is a flood
of error messages about TS read underflow.  Solution: clear the
tx_started flag when the TS is stopped.

Change-Id: I28ff68208f1d3d20dc57c5306fec74bd2f884cb1

mux_demux.c cosmetic: fix indent level in _e1_tx_raw()

Change-Id: Ie0674d6500c9f66b58a7fca8bf28dada035d28d0

mux from not-started raw TS: fill with 0xFF

When the channelized mux reads from a raw TS, but that raw TS is
not in tx_started state yet, the read function returns "fake" data.
However, it was actually returning uninitialized memory content,
rather than 0xFF filler used everywhere else, thereby transmitting
uninit-memory garbage to whatever is connected to the E1 line.
Change it to fill with 0xFF, same as the filler used in other cases
such as inactive timeslots.

Change-Id: I42849a6d19b020bab789853c3b60af6a1c09f92f