open pam sessions with right user and remove setusercontext shim

open pam sessions with right user and remove setusercontext shim

before this change the sessions were opened as the user running doas.
Now it sets its uid to root and then opens a pam session for the target
user.
The setusercontext shim was removed, because pam handles all this and
its easier to just call setresuid and setresgid instead.