Fixes #412.

Due to the Python interpreter's notoriously slow start up time, I opted to read/parse the Magic-Folder API port directly from the api_client_endpoint file (added by tahoe-lafs/magic-folder#593) rather than calling and parsing the output of the magic-folder show-config subcommand. On my modest system, the amount of time saved by not shelling out to magic-folder ends up being quite significant:

% time magic-folder show-config > /dev/null            
magic-folder show-config > /dev/null  0.71s user 0.08s system 98% cpu 0.804 total
% time cat .magic-folder/api_client_endpoint > /dev/null
cat .magic-folder/api_client_endpoint > /dev/null  0.00s user 0.00s system 77% cpu 0.001 total