When adding unprivileged nspawn support we don't really want a global
lock file, since we cannot even access the dir they are stored in, hence
make the concept optional.
Some minor other modernizations.
This is just like lock_generic(), but applies the lock with a timeout.
This requires jumping through some hoops by executing things in a child
process, so that we can abort if necessary via a timer. Linux after all
has no native way to take file locks with a timeout.
Daan De Meyer
A. Wilcox
Lennart Poettering