Issues were:
# github.com/rfjakob/gocryptfs/contrib/findholes/holes
contrib/findholes/holes/holes.go:136:2: unreachable code
# github.com/rfjakob/gocryptfs/tests/root_test_test
tests/root_test/root_test.go:139:2: unreachable code
Also make sure we actually run "go vet" against the whole
codebase.
Also fix incomplete uid restoration in TestSupplementaryGroups
and replace syscall.Setregid and friends with unix.Setregid and
friends.
This test is added to check if have problems handling a full disk.
The ticket https://github.com/rfjakob/gocryptfs/issues/550 states
that the full disk was not where the backing gocryptfs filesystem
was, but this has no effect on gocryptfs, so we test the harder
case.