Fix blocked signals after fork(2)ing external command in subshell

When the classic fork/exec mechanism was used (via sh_fork()) to
run an external command from within a non-forking subshell, SIGINT
was blocked until that subshell was exited. If a subsequent loop
was run in the subshell, it became uninterruptible, e.g.:

   $ arch/*/bin/ksh -c '(/usr/bin/true; while :; do :; done); exit'
   ^C^C^C^C^C

src/cmd/ksh93/sh/xec.c:
- sh_fork() did not reset the savesig variable in the parent part
  of the fork when running in a virtual subshell. This had the
  effect of delaying signal handling until exiting the subshell.
  There is no reason for that subshell check that I can discern, so
  this removes it.
      I've verified that this causes no regression test failures
  even when ksh is compiled with -DSHOPT_SPAWN=0 which means the
  classic fork/exec mechanism is always used.

Fixes: https://github.com/ksh93/ksh/issues/86

NEWS

@@ -3,6 +3,12 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-07-29:
+
+- On a ksh compiled to use fork(2) to run external commands, a bug has been
+  fixed that caused signals (such as SIGINT, Ctrl+C) to be ignored within a
+  non-forked subshell after running an external command within that subshell.
+
 2020-07-25:
 
 - Fixed BUG_MULTIBIFS: Multibyte characters can now be used as IFS