Fix subshell trap integrity, e.g. re-trapping a signal in subshell

Ksh handles local traps in virtual subshells the same way as local
traps in ksh-style shell functions, which can cause incorrect
operation.

Reproducer script:

	trap 'echo "parent shell trap"; exit 0' USR1
	(trap 'echo "subshell trap"' USR1; kill -USR1 $$)
	echo wrong

Output on every ksh93 version: 'wrong'
Output on every other shell: 'parent shell trap'

The ksh93 output is wrong because $$ is the PID of the main shell,
therefore 'kill -USR1 $$' from a subshell needs to issue SIGUSR1 to
the main shell and trigger the 'echo SIGUSR1' trap.

This is an inevitable consequence of processing signals in a
virtual subshell. Signals are a process-wide property, but a
virtual subshell and the parent shell share the same process.
Therefore it is not possible to distinguish between the parent
shell and subshell trap.

This means virtual subshells are fundamentally incompatible with
receiving signals. No workaround can make this work properly.

Ksh could either assume the signal needs to be caught by the
subshell trap (wrong in this case, but right in others) or by the
parent shell trap. But it does neither and just gives up and does
nothing, which I suppose is the least bad way of doing it wrong.

As another example, consider a subshell that traps a signal, then
passes its own process ID (as of 9de65210, that's ${.sh.pid}) to
another process to say "here is where to signal me". A virtual
subshell will send it the PID that it shares with the the parent
shell. Even if a virtual subshell receives the signal correctly, it
may fork mid-execution afterwards, depending on the commands that
it runs (and this varies by implementation as we fix or work around
bugs). So its PID could be invalidated at any time.

Forking a virtual subshell at the time of trapping a signal is the
only way to ensure a persistent PID and correct operation.

src/cmd/ksh93/bltins/trap.c: b_trap():
- Fork when trapping (or ignoring) a signal in a virtual subshell.
  (There's no need to fork when trapping a pseudosignal.)

src/cmd/ksh93/tests/signal.sh:
- Add tests. These are simplified versions of tests already there,
  which issued 'kill' as a background job. Currently, running a
  background job causes a virtual subshell to fork before forking
  the 'kill' background job (or *any* background job, see e3d7bf1d)
  -- an ugly partial workaround that I believe just became
  redundant and which I will remove in the next commit.

NEWS

@@ -3,6 +3,15 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-06-13:
+
+- Trapping a signal that is not a pseudosignal will now cause a virtual
+  subshell to fork into a real one. This ensures a persistent PID where
+  other processes can signal the subshell (the PID of a virtual subshell may
+  change as other commands cause it to fork), and fixes a bug where a signal
+  could not be correctly or effectively trapped in a subshell if the same
+  signal was also trapped in the main shell environment.
+
 2022-06-12:
 
 - The POSIX mode now disables zero-padding of seconds in 'time'/'times' output.