init: Add timeout when locking mapset, add locking to Python API

[wenzeslaus]

This change introduces two things:

1. Timeout parameter in CLI which is enabled by default and makes GRASS wait if the mapset is locked.
2. Lock parameter to the grass.script.setup.init function so that Python API can now lock the mapset if desired.

Additionally, this syncs locking parameters and underlying code between CLI and Python API so both timeout and forcible removal of lock are available in Python as well. The tests are using the Python API.

Locked mapset example in CLI:

$ grass ~/grassdata/nc_spm_08_grass7/user1/
Starting GRASS GIS...
Mapset <.../nc_spm_08_grass7/user1> locked (attempt 1), but will retry in 1.00 seconds...
Mapset <.../nc_spm_08_grass7/user1> locked (attempt 2), but will retry in 2.00 seconds...
Mapset <.../nc_spm_08_grass7/user1> locked (attempt 3), but will retry in 4.00 seconds...

Lock in Python (also works as a context manager):

import grass.script as gs
s = gs.setup.init("~/grassdata/nc_spm_08_grass7/user1/", lock=True, timeout=0)
s.finish()

This also introduces an experimental execution of the grass.app with CLI API using python -m grass.app. Specifically, lock and unlock subcommands are now available and can be used for testing, fixing lock situations, and by external applications which need to persistently lock mapset without using the Python API directly.

Example lock-unlock (path to Python packages needs to be setup, don't ask me why LD_LIBRARY_PATH does not need to be setup - it is needed for test of the Python API):

export PYTHONPATH="$(.../bin.x86_64-pc-linux-gnu/grass --config python-path)"
python -m grass.app lock ~/grassdata/nc_spm_08_grass7/user1/
python -m grass.app unlock ~/grassdata/nc_spm_08_grass7/user1/

[wenzeslaus]

I need to fix the tests...there is no locking on Windows. Otherwise the tests pass.