globalpackages: add enable/disable/toggle subcommands, add help

[airwoodix]

Hello!

This patch implements explicit actions for vf globalpackages as discussed in #181. The old behavior (toggle) is preserved when calling with no explicit action. I also added a -q/--quiet switch to remove the new state display. Finally, reworking this was the opportunity to add a help message, following the incentive in #105.

New usage:

• vf globalpackages [-q]: toggle use of system site packages
• vf globalpackages toggle [-q]: same as above
• vf globalpackages enable [-q]: enable system site packages
• vf globalpackages disable [-q]: disable system site packages

The command returns 0 up success.

Comments most welcome. Thanks!

[airwoodix]

Thanks for the feedback! Using long options sounds good to me. I'd go for the short version if there's something widely used/idiomatic but that's definitely not the case here. Sorry for the faulty copy-paste (I guess leaving the duplicated comment is good to avoid messing up with the next line at all spots).