Building on Illumos Issue

[Smithx10]

Illumos doesn't have sys call.Flock. Could we please use syscall.FcntlFlock ?

# code.gitea.io/git/vendor/gopkg.in/src-d/go-billy.v4/osfs
../git/vendor/gopkg.in/src-d/go-billy.v4/osfs/os_posix.go:13:9: undefined: syscall.Flock
../git/vendor/gopkg.in/src-d/go-billy.v4/osfs/os_posix.go:13:41: undefined: syscall.LOCK_EX
../git/vendor/gopkg.in/src-d/go-billy.v4/osfs/os_posix.go:20:9: undefined: syscall.Flock
../git/vendor/gopkg.in/src-d/go-billy.v4/osfs/os_posix.go:20:41: undefined: syscall.LOCK_UN

[Smithx10]

https://gitlab.cncf.ci/kubernetes/kubernetes/blob/3412f5b8eb3b3d9991d035193d5989f250343e3f/third_party/forked/etcd237/pkg/fileutil/lock_solaris.go

[Smithx10]

@mcuadros In the next release of golang, Illumos will have its own build tag... So I believe taking advantage of the native flock system calls should be possible then.