Add the -L and -l options to install. -L allows an alternative /etc directory

Add the -L and -l options to install.  -L allows an alternative /etc directory
path to be specified and causes install to search LOPT/group and
LOPT/master.passwd for group and user names.  If the -l option is also
specified, install will fallback to system files if the group or user name
cannot be found in the alternative /etc directory.

This is intended to allow us to ensure that the correct uid's and gid's are
used in a release build and to allow new uid's and gid's to be used during
an installworld before they have necessarily been added to the system
password and group files.