Fix an infinite loop related to $_ if ksh is /bin/sh (#90)

The following explanation is mostly taken from Tomas Klacko's report on
the old mailing list (which also contains a C program reproducer) [*]:

1. When ksh starts a binary, it sets its environment variable "_"
   to "*number*/path/to/binary". Where "number" is the pid of the
   ksh process.

2. The binary forks and the child executes a suid root shell script
   which begins with #!/bin/sh. For this bug to occur, ksh must be /bin/sh.

3. The ksh process interpreting the suid shell script leaves the "_"
   variable as not set (nv_getval(L_ARGNOD) returns NULL) because
   the "number" from step 1 is not the pid of its parent process.

4-5. Because "_" is not set and the script is suid root, an infinite
   loop occurs because when the SHELL environment variable contains
   "/bin/sh" pathshell() returns "/bin/sh". This becomes an infinite
   loop of /bin/sh /dev/fd/3 executing /bin/sh /dev/fd/3.

src/cmd/ksh93/sh/init.c: get_lastarg():
- Disable the check for if the "number" refers to the process id of
  the parent process.

src/cmd/ksh93/sh/main.c: sh_main():
- Prevent an infinite loop when '$_' is not passed in from the environment.

Solaris applies this bugfix to their version of ksh:
https://github.com/oracle/solaris-userland/blob/master/components/ksh93/patches/190-17432413.patch

[*]: https://www.mail-archive.com/ast-developers@lists.research.att.com/msg01680.html

NEWS

@@ -5,6 +5,8 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 
 2020-07-23:
 
+- Fixed an infinite loop that could occur when ksh is the system's /bin/sh.
+
 - A command substitution that is run on the same line as a here-document
   will no longer cause a syntax error.