COMMENT(!MOD!zsh/clone
A builtin that can clone a running shell onto another terminal.
!MOD!)
The tt(zsh/clone) module makes available one builtin command:

startitem()
findex(clone)
cindex(shell, cloning)
cindex(cloning the shell)
cindex(terminal)
item(tt(clone) var(tty))(
Creates a forked instance of the current shell, attached to the specified
var(tty).  In the new shell, the tt(PID), tt(PPID) and tt(TTY) special
parameters are changed appropriately.  tt($!) is set to zero in the new
shell, and to the new shell's PID in the original shell.

The return status of the builtin is zero in both shells if successful,
and non-zero on error.

The target of tt(clone) should be an unused terminal, such as an unused virtual
console or a virtual terminal created by

xterm -e sh -c 'trap : INT QUIT TSTP; tty; while :; do sleep 100000000; done'

Some words of explanation are warranted about this long xterm command
line: when doing clone on a pseudo-terminal, some other session
("session" meant as a unix session group, or SID) is already owning
the terminal. Hence the cloned zsh cannot acquire the pseudo-terminal
as a controlling tty. That means two things:

      the job control signals will go to the sh-started-by-xterm process
      group (that's why we disable INT QUIT and TSTP with trap; otherwise
      the while loop could get suspended or killed)

      the cloned shell will have job control disabled, and the job
      control keys (control-C, control-\ and control-Z) will not work.

This does not apply when cloning to an bf(unused) vc.

Cloning to an used (and unprepared) terminal will result in two
processes reading simultaneously from the same terminal, with
input bytes going randomly to either process.

tt(clone) is mostly useful as a shell built-in replacement for
openvt.
)
enditem()
