Jungerl: bugfix in the posix_drv driver

[Scott Lystig Fritchie]

For what it's worth...

... I fixed a significant bug in the getgrnam() and getgrgid()
portions of the posix_drv driver (in the Jungerl collection).

POSIX functions supported by the driver:

    getegid geteuid getgid getgrgid getgrnam getgroups getlogin getpgrp
    getppid getpwnam getpwuid getsid getuid kill lstat mkfifo mknod umask

If there are other POSIX'ish functions that aren't currently supported
by an OTP library somewhere but you'd like to see it implemented, let
me know.  I've intentionally avoided implemented system calls such as
setuid(), but if someone wants them, they're easy to add.

-Scott