lavfi/volume: support volume expression and per-frame expression evaluation

The eval mode allows to evaluate the expression per-frame or just at
init.

In particular, address ticket #3234.
lavfi/volume: implement process_command() callback, with the volume command

Address trac ticket #2868.
lavfi/volume: extend volume logging
doc/protocols/udp,lavf/udp: apply minor documentation fixes
lavf/tcp: honor listen option value in a tag

Consistent with what is done in udp.c, allow to explicitly disable listen
with listen=0.
doc/protocols/tcp,lavf/tcp: apply minor fixes to TCP protocol documentation
lavfi/volume: fix NULL reference in filter_frame()

Fix crash.
ffprobe: Dont clear AVFrame between uses.

The old API required this clearing in the past, the new API does not
require it.

Fixes memleak
Regression introduced by 37a749012aaacc801fe860428417a6d7b81c103f

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>