add run-as to run hook in other user context

[malaupa]

This PR introduces the use of syscall.SysProcAttr.Credential to set uid and gid. These values are queried using os/user.Lookup with a given username.

It would be nice to have this option available, for instance, file operations with different user context than root is often necessary.

[adnanh]

Hey, thank you for your contribution! This should already be possible to achieve using the -setuid and -setgid flags.