Noticket: [factorio_manager.sh] Expand checks for bool/int values to achieve better correctness of the code:

A && B || C in Bash is not equivalent to:

if A; then
B
else
C
fi

In fact, if A is true, and B is false, C will still get run.

In this particular case it's not a problem, but better to get rid of
it and have slightly longer code.

Noticket: [factorio_manager.sh] Use quotes around right-hand sides of operators:

- Right-hand side is treated as glob, so quoting is necessary to avoid
errors. For example, if var='[a]', then [[ $a == $a ]] will always
be false.

Noticket: Deduplicate locking code in Factorio Manager:

- Introduce helper function for cleaning-up any registered locks.
- Introduce helper function for calling flock that exits in case of an
error as well.
- Make existing code use the new locking function.