Make os.exec support supplementary groups

[bnoordhuis]

Add a .groups property that is an array of group ids for setgroups.

Fixes: #1055

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No test because it cannot be tested without elevated privileges but strace -fe setgroups shows it works with both empty and non-empty lists:

$ strace -fe q=attach,exit -fe signal=kill -fe setgroups build/debug/qjs --std -e 'os.exec(["true"], {groups:[]})'
[pid 87228] setgroups(0, [])            = 0

$ strace -fe q=attach,exit -fe signal=kill -fe setgroups build/debug/qjs --std -e 'os.exec(["true"], {groups:[1,2,3]})'
[pid 87252] setgroups(3, [1, 2, 3])     = 0