Autotoolize build system

Set PLUGINDIR and CONFDIR in janus.c and janus.cfg.
Download .alaw files and commit them into git.
Allow disabling individual plugins.
janus-pp-rec: Fix remuxing of opus streams.

The sequence number is a short, when it "overflows" we need to increment
a "steps" multiplier that will be used in the position calculation.

last_seq was not updated when updating steps, which led it to be
incremented at each loop cycle, which led to incorrect position
calculation.
Convert sessions watchdog to use a glib mainloop