setuid and setgid accept string as a parameter.

Both process.setuid and process.setgid can accept either a numerical ID or a username\groupname string.
Added overloaded version of setgid and setuid.

Instead of making argument's type 'any' we have overloaded version for number and string parameters.
Merge pull request #469 from AntonAM/patch-1

setuid and setgid accept string as a parameter.