Fix argv rewrite on invoking hashbangless script (rhbz#1047506)

The fixargs() function is invoked when ksh needs to run a script
without a #!/hashbang/path. Instead of letting the kernel invoke a
shell, ksh exfile()s the script itself from sh_main(). In the
forked child, it calls fixargs() to set the argument list in the
environment to the args of the new script, so that 'ps' and
/proc/PID/cmdline show the expected output.

But fixargs() is broken because, on systems other than HP-UX (on
which ksh uses pstat(2)), ksh simply inserts a terminating zero.
The arguments list is not a zero-terminated C string. Unix systems
expect the entire arguments buffer to be zeroed out, otherwise 'ps'
and /proc/*/cmdline will have fragments of previous command lines
in the output.

The Red Hat patch for this bug is:
642af4d6/f/ksh-20120801-argvfix.patch

However, that fix is incomplete because 'command_len' was also
hardcoded to be limited to 64 characters (!), which still gave
invalid 'ps' output if the erased command line was longer.

src/cmd/ksh93/sh/main.c: fixargs():

- Remove CMD_LENGTH macro which was defined as 64.

- Remove code that limited the erasure of the arguments buffer to
  CMD_LENGTH characters. That code also had quite a dodgy strdup()
  call -- it copies arguments to the heap, but they are never freed
  (or even used), so it's a memory leak. Also, none of this is
  ever done if the length is calculated using pstat(2) on HP-UX,
  which is a clear indication that it's unnecessary.
  (I think this code block must have been some experiment they
  forgot to remove. One reason why I think so is that a 64 byte
  arguments limit never made sense, even in the 1980s when they
  wrote ksh on 80-column CRT displays. Another indication of this
  is that fixing it didn't require adding anything; the code to do
  the right thing was already there, it was just being overridden.)

- Zero out the full arguments length as in the Red Hat patch.

src/cmd/ksh93/tests/basic.sh:

- Add test. It's sort of involved because 'ps' is one of the least
  portable commands in practice, in spite of standardisation.

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-09-24"
+#define SH_RELEASE	"93u+m 2020-09-25"