remove dependency on `mount` binary

[AkihiroSuda]

https://github.com/rootless-containers/rootlesskit/blob/master/pkg/child/child.go has extra exec for mount(8) binary.

mount(2) syscall could be called directly.

[Zyqsempai]

I will take this one;)

[AkihiroSuda]

thanks

[AkihiroSuda]

We still have:

$ git grep \"mount\"
pkg/child/hosts.go:             {"mount", "--bind", myEtcHosts, "/etc/hosts"},
pkg/child/resolvconf.go:                {"mount", "--bind", myResolvConf, "/etc/resolv.conf"},
pkg/copyup/tmpfssymlink/tmpfssymlink.go:                        {"mount", "--rbind", d, bind0},
pkg/copyup/tmpfssymlink/tmpfssymlink.go:                        {"mount", "-n", "-t", "tmpfs", "none", d},
pkg/copyup/tmpfssymlink/tmpfssymlink.go:                        {"mount", "-n", "--move", bind0, bind1},

[Zyqsempai]

@AkihiroSuda Will take care of it on next week if you don't mind, I am on vacation.

[AkihiroSuda]

thanks