define ports by amount, not individually - v2

[qk-santi]

Defining the number of ports for transfer connections is a bit cumbersome, especially if you want to create quite a few ones

croc send --ports 9009,9010,9011,9012,9013,9014,9015,9016,9017 my_file.gzip

Switched to:

Define base port (if different than default) + number of parallel connections
croc send --port 9009 --transfers 8 my_file.gzip
croc send --transfers 8 my_file.gzip

Fixes #617

Related to PR #660

[schollz]

this works, thanks!!

[jathek]

Could this be added to the relay command as well? That still uses --ports and requires manually typing each one.