envuidgid — runs another program with environment variables indicating a specified account's uid and gid
envuidgid {account} {child}
account is a single argument.
child consists of one or more arguments.
envuidgid sets $UID to account's uid and $GID to account's gid.
It then runs child.
envuidgid exits 111 if it cannot find a UNIX account named account,
if it runs out of memory for environment variables, or if it cannot run child.
Otherwise its exit code is the same as that of child.