osmux: Split input and output code into separate files

The output and input paths are totally independent, they share no code
or structures holding state. They can be operated totally independently.
Hence, let's split the code into separate file since when someone looks
at the osmux code one is really looking specifically at Tx or Rx side,
but not at both sides.
Moreover, the "input" and "output" concepts have always been a bit
difficult to understand (would have been better calling them "src" and
"sink" instead), which adds more confusion when trying to find the
relevant part of code.

Change-Id: Ia72995092a36ca50147611e617cb88c4dcf231d5
---
M src/Makefile.am
M src/osmux.c
A src/osmux_input.c
A src/osmux_output.c
4 files changed, 1,065 insertions(+), 959 deletions(-)