add logfile logic and cli flag

[netzego]

add a logic for logfile and actually do the logging (to a file). also add --log and --logfile flag and option to set logging behavior via cli arguments.

[netzego]

LOGFILE must be available outside of main. but load_configfile() and parse_args() are called inside main. this need a refactor to work.

[netzego]

• don't forget to check if LOGFILE exist
• write func check_logfile(); check_logfile.bash
• and test 20-check_logfile.bats

[netzego]

do we need LOG and LOGFILE? or can we just use LOGFILE? maybe introduce LOGFILE also in vars.bash.

• default var vars.bash; LOGFILE=/dev/null
• config var SCRIPTNAME.config; optional; could be absent (#LOGFILE=...)
• cliargs LOGFILE; optional