Add --functrace shell option (re: 2a835a2d)

A side effect of the bug fixed in 2a835a2d caused the DEBUG trap
action to appear to be inherited by subshells, but in a buggy way
that could crash the shell. After the fix, the trap is reset in
subshells along with all the others, as it should be. Nonetheless,
as that bug was present for years, some have come to rely on it.

This commit implements that functionality properly. When the new
--functrace option is turned on, DEBUG trap actions are now
inherited by subshells as well as ksh function scopes. In addition,
since it makes logical sense, the new option also causes the
-x/--xtrace option's state to be inherited by ksh function scopes.
Note that changes made within the scope do not propagate upwards;
this is different from bash.

(I've opted against adding a -T short-form equivalent as on bash,
because -T was formerly a different option on 93u+ (see 63c55ad7)
and on mksh it has yet anohter a different meaning. To minimise
confusion, I think it's best to have the long-form name only.)

src/cmd/ksh93/include/shell.h,
src/cmd/ksh93/data/options.c:
- Add new "functrace" (SH_FUNCTRACE) long-form shell option.

src/cmd/ksh93/sh/subshell.c: sh_subshell():
- When functrace is on, copy the parent's DEBUG trap action into
  the virtual subshell scope after resetting the trap actions.

src/cmd/ksh93/sh/xec.c: sh_funscope():
- When functrace is on and xtrace is on in the parent scope, turn
  it on in the child scope.
- Same DEBUG trap action handling as in sh_subshell().

Resolves: https://github.com/ksh93/ksh/issues/162

ANNOUNCE

@@ -70,6 +70,12 @@ New command line editor features:
 
 New features in shell options:
 
+- A new --functrace long-form shell option causes the -x/--xtrace option's
+  state and the DEBUG trap action to be inherited by function scopes instead
+  of being reset to default. Changes made to them within a function scope
+  still do not propagate back to the parent scope. Similarly, this option
+  also causes the DEBUG trap action to be inherited by subshells.
+
 - The new --histreedit and --histverify options modify history expansion
   (--histexpand). If --histreedit is on and a history expansion fails, the
   command line is reloaded into the next prompt's edit buffer, allowing