Global flock active by default (but shouldn't)

[buhtz]

Context: Bug was introduce by me with PR #1697 providing a new GlobalFlock context manager.

Problem: The check of the config value global.use_flock is gone. So all user will have an activate global flock no matter of the configuration.

Before PR #1697 that global flock was disabled by default config. That config value was queried in a method named flockExclusive().

backintime/common/snapshots.py

Line 784 in 4d7e920

# self.flockExclusive()

Solution:
My proposed solution is to add a new argument to GlobalFlock() context manager to disable it based on the config value global.use_flock.
That is not elegant but there is a structure problem with the code in snapshots.py using that context manager. The code block handled by the context manager has hundred of lines of code. Because of that I can not do something like this:

if cfg.globalFlock():
    with flock.GlobalFlock():
        do_the_magic()
else:
    do_the_magic()

I'll combine the solution together with Issue #1743