Modify run_ffmpeg() & add 3 append_*() functions.
Modify run_ffmpeg() & add 3 append_*() functions.

Add support for empty logfile name into run_ffmpeg() function. Don't
die() at verbosity level 0.

Add append_ins2cmd(), append_ingrps2cmd() and append_tgrp2cmd()
functions and refactor ffmpeg calling code in stream detection,
subtitles extraction, volnorm and video encoding phases.

Use verbosity 1 when calling run_ffmpeg() during subtitles extraction
phase.

Look for executable ffmpeg, bc and perl in PATH before running any
phases.

xrgtn/recode-vid,xrgtn/recode-vid