Move setting of default core_pid=0 into _parse_arguments()

_parse_arguments() is now the only place where $core_pid is set and then
remains constant for the entire script execution. Improves code
readability.
Move setting of default $core_sig into _parse_arguments()

_parse_arguments() is now the only place where $core_sig is set and then
remains constant for the entire script execution. Improves code
readability.
Don't check free space when we don't have valid core pid

Previous condition could cause the script to hang on 'cat > /dev/null'
when invoked from Quick Feedback to collect only logs and device-side
stacktrace was enabled in settings. Without a PID, we don't care much
about free space anyway.
Make core location path a constant
Refactor core location check

Also prevent script getting stuck when $core_pid -eq 0 (empty stdin)
Override disabled coredumping when dump is invoked from Quick Feedback
Merge pull request #31 from xhaakon/prevent-quickie-hangs

Prevent rich-core-dumper getting stuck when invoked by Quick Feedback in order to just collect logs