set -b/--notify: do not mess up the command line

This commit makes three interrelated changes.

First, the code for erasing the command line before redrawing it
upon a window size change is simplified and modernised. Instead of
erasing the line with lots of spaces, it now uses the sequence
obtained from 'tput ed' (usually ESC, '[', 'J') to "erase to the
end of screen". This avoids messing up the detection and automatic
redrawing of wrapped lines on terminals such as Apple Terminal.app.

Second, the -b/--notify option is made more usable. When it is on
and either the vi or emacs/gmacs line editor is in use, 'Done' and
similar notifications are now buffered and trigger a command line
redraw as if the window size changed, and the redraw routine prints
that notify buffer between erasing and redrawing the commmnd line.
The effect is that the notification appears to magically insert
itself directly above the line you're typing. (The notification
behaviour when not in the shell line editor, e.g. while running
commands such as external editors, is unchanged.)

Third, a bug is fixed that caused -b/--notify to only report on one
job when more than one terminated at the same time. The rest was
reported on the next command line as if -b were not on. Reproducer:
$ set -b; sleep 1 & sleep 1 & sleep 1 &

This commit also includes a fair number of other window size and
$COLUMNS/$LINES handling tweaks that made all this easier, not all
of which are mentioned below.

src/cmd/ksh93/include/fault.h,
src/cmd/ksh93/sh/fault.c:
- Replace sh_update_columns_lines with a new sh_winsize() function.
  It calls astwinsize() and is to be used instead of it, and
  instead of nv_getval(LINES) and nv_getval(COLUMNS) calls. It:
  - Allows passing one or neither of lines or cols pointers.
  - Updates COLUMNS and LINES, but only if they actually changed
    from the last values. This makes .set discipline functions
    defined for these variables more useful.
  - Sets the sh.winch flag, but only if COLUMNS changes. If only
    the height changes, the command line does not need redrawing.

src/cmd/ksh93/include/io.h:
- Add sh_editor_active() that allows checking whether one of vi,
  emacs or gmacs is active without onerous #if SHOPT_* directives.

src/cmd/ksh93/sh/jobs.c: job_reap():
- Remove the fix backported in fc655f1a, which was really just a
  workaround that papered over the real bug.
- Move a check for errno==ECHILD so it is only done when waitpid()
  returns an error (pid < 0); the check was not correct because C
  library functions that do not error out also do not change errno.
- Move the SH_NOTIFY && SH_TTYWAIT code section to within the
  while(1) loop so it is run for each job, not only the
  last-processed one. This fixes the bug where only one job was
  notified when more than one ended at the same time.
- In that section, check if an editor is active; if so, set the
  output file for job_list() to sh.notifybuffer instead of standard
  error, list the jobs without the initial newline (JOB_NLFLAG),
  and trigger a command line redraw by setting sh.winch.

src/cmd/ksh93/edit/edit.c:
- Obtain not just CURSOR_UP but also ERASE_EOS (renamed from
  KILL_LINE) using 'tput'. The latter had the ANSI sequence
  hardcoded. Define a couple of TPUT_* macros to make it easier to
  deal with terminfo/termcap codes.
- Add get_tput() to make it easier to get several tput values
  robustly (with SIGINT blocked, trace disabled, etc.)
- ed_crlf(): Removed. Going by those ancient #ifdefs, nothing that
  93u+m will ever run on requires a '\r' before a '\n' to start a
  new line on the terminal. Plus, as of 93u+, there were already
  several spots in emacs.c and vi.c where it printed a sole '\n'.
- ed_read():
  - Simplify/modernise command line erase using ERASE_EOS.
  - Between erasing and redrawing, print the contents of the notify
    buffer. This has the effect of inserting notifications above
    the command line while the user is typing.

src/cmd/ksh93/features/cmds:
- To detect terminfo vs termcap codes, use all three codes we're
  currently using. This matters on at least on my system (macOS
  10.14.6) in which /usr/bin/tput has incomplete terminfo support
  (no 'ed') but complete termcap support!

src/cmd/ksh93/include/edit.h

@@ -174,7 +174,6 @@ typedef struct edit
 		(c<'J'?c+1-'A':(c+10-'J'))))))))))))))))
 #endif
 
-extern void	ed_crlf(Edit_t*);
 extern void	ed_putchar(Edit_t*, int);
 extern void	ed_ringbell(void);
 extern void	ed_setup(Edit_t*,int, int);