sh6i alias subshell - trap 2(INT) 3(QUIT) 15(TERM)

Trapping these signals in the first subshell and untrapping them
in the second (or nested) subshell before osh invokes sh6 allows
the first subshell to survive common user actions in sh6.  EG:

% ps To ppid,pid,pgid,tty,stat,command
 PPID   PID  PGID TTY      STAT COMMAND
  532 13654 13654 ttys003  Ss+  -osh
13654 16262 13654 ttys003  S+   -osh
16262 16334 13654 ttys003  S+   sh6
16334 16491 13654 ttys003  R+   ps To ppid,pid,pgid,tty,stat,command
% ^C^C^C
% ^\^\^\
% kill $$
% kill 0
Sig 15
% ps To ppid,pid,pgid,tty,stat,command
 PPID   PID  PGID TTY      STAT COMMAND
  532 13654 13654 ttys003  Ss+  -osh
13654 16262 13654 ttys003  S+   -osh
16262 16334 13654 ttys003  S+   sh6
16334 16494 13654 ttys003  R+   ps To ppid,pid,pgid,tty,stat,command

...
Process ID 16262 is the one we protect in this example.