Fix crash: do not list jobs if there is no job control

This bug caused an undefined state, which sometimes crashed the
shell in job_list() or job_unpost(), if $PS1 contains a command
substitution running an external command and the '-b'/'-o notify'
shell option is active. So far the only known way to trigger the
crash is by letting $TMOUT time out the interactive shell. See
https://github.com/ksh93/ksh/issues/103 for details.

src/cmd/ksh93/sh/jobs.c: job_reap():
- The check for the SH_NOTIFY option and the SH_TTYWAIT state
  before listing jobs was insufficient. Job control is disabled in
  command substitutions, so also check that job control is active
  before listing jobs.

src/cmd/ksh93/sh.1:
- Fix TMOUT documentation. The 'read' command in fact only times
  out when reading from a terminal, just like 'select'. Also
  document the extra 60 second grace period when an interactive
  shell prompt reads from a terminal.

Fixes: https://github.com/ksh93/ksh/issues/103

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-08-06:
+
+- Fixed a crash that occurred intermittently if 'set -b'/'set -o notify' is
+  active and $PS1 contains a command substitution running an external command.
+
 2020-08-05:
 
 - Fixed a bug in functions that caused ksh to crash when an array with an