script: support sig{stop/cont}

script: support sig{stop/cont}

* call wait() only when child exited
* suspend all session (including script master process) when child get
  SIGSTOP and send SIGCONT to child when master process resume

This allows to suspend all session and later use "fg" shell command to
resume.

$ ps af
14722 pts/1    Ss     0:00 bash
 4870 pts/1    S+     0:00  \_ ./script
 4871 pts/6    Ss+    0:00      \_ bash -i

$ kill -SIGSTOP 4871

and script session on another terminal:

$ script
Script started, file is typescript
$ 
[1]+  Stopped                 ./script

$ fg 1
./script

... session again usable ...
^D
Script done, file is typescript

Signed-off-by: Karel Zak <kzak@redhat.com>