Do not exit shell on Ctrl+C with SIGINT ignored (re: 7e5fd3e9)

The killpg(getpgrp(),SIGINT) call added to ed_getchar() in that
commit caused the interactive shell to exit on ^C even if SIGINT is
being ignored. We cannot revert or remove that call without
breaking job control. This commit applies a new fix instead.

Reproducers fixed by this commit:

  SIGINT ignored by child:

    $ PS1='childshell$ ' ksh
    childshell$ trap '' INT
    childshell$ (press Ctrl+C)
    $

  SIGINT ignored by parent:

    $ (trap '' INT; ENV=/./dev/null PS1='childshell$ ' ksh)
    childshell$ (press Ctrl+C)
    $

  SIGINT ignored by parent, trapped in child:

    $ (trap '' INT; ENV=/./dev/null PS1='childshell$ ' ksh)
    childshell$ trap 'echo test' INT
    childshell$ (press Ctrl+C)
    $

I've experimentally determined that, under these conditions, the
SFIO stream error state is set to 256 == 0400 == SH_EXITSIG.

src/cmd/ksh93/sh/main.c: exfile():
- On EOF or error, do not return (exiting the shell) if the shell
  state is interactive and if sferror(iop)==SH_EXITSIG.
- Refactor that block a little to make the new check fit in nicely.

src/cmd/ksh93/tests/pty.sh:
- Test the above three reproducers.

Fixes: https://github.com/ksh93/ksh/issues/343

NEWS

@@ -9,6 +9,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   command to: basename, cat, cp, cut, dirname, getconf, ln, mktemp, mv.
   Add /opt/ast/bin to your $PATH to use these. Type 'cp --man', etc. for info.
 
+- A bug introduced on 2020-09-17 was fixed that caused interactive ksh to exit
+  if Ctrl+C was pressed while SIGINT was being ignored (as in "trap '' INT").
+
 2021-12-13:
 
 - Fixed a bug introduced on 2020-08-09 that prevented '.' and '..' from