Make username resolution configurable

[kevinreddot]

getpwuid() requires /etc/nsswitch.conf, NSS libraries and, depending on NSS configuration, real /etc/passwd file available. If forsh is to be run in a chrooted environment, those files needs to be put there and always updated with the latest version. This complicates chroot environment setup and maintenance. On the other hand, getuid() call does not require any files and works fine even if no NSS-related files are available.

Make it configurable if forsh calls getpwuid for name resultion.