This change refactors the process group implementation and adds support
for sessions. The setsid(2) and getsid(2) system calls were added.

psctl(2) now has PSCTL_TTYNAME, which lets you get the name of a process's
terminal, and ps(1) now uses it.

The initial terminal is now called /dev/tty1.

/dev/tty is now a factory for the current terminal.

A global lock now protects the process hierarchy which makes it safe to
access other processes. This refactor removes potential vulnerabilities
and increases system robustness.

A number of terminal ioctls have been added.

This is a compatible ABI change.