Ability to specify supplementary groups in bluepill

[pilbot]

It would be useful to have the option of bluepill applications and processes having:

• The default groups of a user
• Ability to specify the supplementary groups of a process

If we have a user user1 who has a primary group groupa and supplementary groups groupb and groupc. And a few files:

-rw-r----- 1 user1    groupa  1 Jan  8 10:30 file1
-rw-r----- 1 user2    groupb  1 Jan  8 10:30 file2
-rw-r----- 1 user2    groupa  1 Jan  8 10:30 file3
-rw-r----- 1 user2    groupc  1 Jan  8 10:30 file4

In this case the user should be able to read all of the files above as should a process invoked from the users shell. It would be nice to have this functionality in bluepill as we are currently restricted to just specifying the gid which means that we can only access a subset of the files above.