Allow kill built-in to terminate interactive shell

Reproducer:

  $ kill -s HUP $$
  $

Since the signal is not SIGTERM or SIGQUIT and is not trapped, even
the interactive shell should terminate, but ksh happily returns to
the prompt. The exit status reflects that SIGHUP was sent, but the
interactive shell fails to exit when the 'kill' built-in is used.
Oddly enough, the external 'kill' command works fine:

  $ command -x kill -s HUP $$
  Hangup

On every other shell, the kill built-in also works as expected.

Analysis: in sh_fault(), when processing a signal flagged with
SH_DONE (terminates the shell by default), the mode flag in
sh.jmplist (the longjmp return stack) is set to SH_JMPSUB or
SH_JMPFUN, values used for returning from virtual subshells and
ksh-style functions, relatively. But those will not cause the shell
to exit as expected. For that, we need SH_JMPEXIT (see main.c line
628-629).

src/cmd/ksh93/sh/fault.c: sh_fault():
- Make sure the SH_JMPEXIT longjmp return mode is used when the
  shell is in the SH_INTERACTIVE state. (Note that, as of 48ba696,
  this state bit is alwayas off in subshells, so this change does
  not affect subshells of interactive shells.)

NEWS

@@ -9,6 +9,9 @@ Uppercase BUG_* IDs are shell bug IDs as used by the Modernish shell library.
   causing a stopped process to be incorrectly resumed upon sending it any
   signal and causing any SIGCONT trap handler to be incorrectly triggered.
 
+- Fixed: the 'kill' built-in was not able to terminate an interactive shell
+  by sending a signal such as SIGHUP to its own shell (e.g. kill -s HUP $$).
+
 2024-01-05:
 
 - Fixed a potential crash on pressing Ctrl+C on the command line while

src/cmd/ksh93/sh/fault.c

@@ -128,7 +128,7 @@ void	sh_fault(int sig)
 			sigrelease(sig);
 			if(pp->mode != SH_JMPSUB)
 			{
-				if(pp->mode < SH_JMPSUB)
+				if(pp->mode < SH_JMPSUB && !sh_isstate(SH_INTERACTIVE))
 					pp->mode = sh.subshell?SH_JMPSUB:SH_JMPFUN;
 				else
 					pp->mode = SH_JMPEXIT;

src/cmd/ksh93/tests/pty.sh

@@ -1255,5 +1255,17 @@ w _ksh_93u_m_cmdcompl\t
 r :test-2: _ksh_93u_m_cmdcomplete_test_ \r\n$
 !
 
+tst $LINENO <<"!"
+L terminate interactive shell using the kill built-in
+
+d 40
+p :test-1:
+w PS1=':child-!: ' "$SHELL"
+p :child-1:
+w kill -s HUP \$\$
+r ^:child-1: kill -s HUP \$\$\r\n$
+r ^Hangup\r\n$
+!
+
 # ======
 exit $((Errors<125?Errors:125))