Fix network/physical SMF methods to work in absence of /usr filesystem
Currently many routines in /lib/svc/method/net-nwam, net-physical and net-iptun as well as /lib/svc/share/* scripts rely on programs from /usr/bin. Some of these programs may have equivalents in the system shell (ksh93) builtins, others can be replaced with equivalent logic based on shell capabilities.
Due to the currently defined SMF dependencies it is problematic to cause the "network/physical" services to depend on "filesystem/root" (circular dependencies arise), and networking startup fails when /usr is a separate filesystem (see #829) due to unexecutable instructions in the method scripts.
At the moment "physical:default" starts with errors, but works for both static files and DHCP, and "physical:nwam" fails completely, during the initial startup. They do restart (on request) and complete without errors after the OS is fully up and has mounted the separated child filesystems of the rootfs.