Cygwin: workaround for ksh to execute #!-less scripts with itself

On Cygwin, ksh does not execute scripts without a #! path in a fork
of the ksh process as it does on other systems. Reproducer (run
from ksh):

  $ cat test.sh
  echo "${BASH_VERSION:-not bash}"
  echo "${.sh.version}"
  $ chmod +x test.sh
  $ ./test.sh
  4.4.12(3)-release
  ./test.sh: line 2: ${.sh.version}: bad substitution

The script was executed in bash instead of ksh.

After this fix, the output on Cygwin is like ksh on other systems:

  not bash
  Version AJM 93u+m/1.1.0-alpha+dev 2022-01-26

This also fixes a number of regression test failures, as quite a
few tests create and execute temp scripts without a hashbang path.

Analysis: On Cygwin, execve(2) happily executes shell scripts
without a #! path with /bin/sh (which is bash --posix). However,
ksh relies on execve(2) executing binaries or #! only, as it uses
an ENOEXEC failure to decide whether to fork and execute a #!-less
shell script with a reinitialized copy of itself via exscript().

src/cmd/ksh93/sh/path.c: path_spawn():
- Look at the magic first two bytes of the file; if it is "MZ"
  (Mark Zbikowski, originator of the .exe format) or "#!", continue
  as normal, otherwise simulate an ENOEXEC failure from execve(2)
  which will cause ksh to fall back on #!-less script execution.

NEWS

@@ -3,6 +3,11 @@ 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.
 
+2022-01-26:
+
+- On Cygwin, ksh now executes scripts that do not have a #! path itself,
+  like it does on other systems, instead of with /bin/sh.
+
 2022-01-24:
 
 - Fixed a crashing bug in history expansion that could occur when using the "&"