Remove ineffective check for login shell; require -p for suid/sgid

In main.c:

158:		if(sh.ppid==1)
159:			sh.login_sh++;

If that was ever valid, it certainly is not now. As far as I know,
there is no currently existing system where PID 1 (init or systemd
or whatever) is the parent shell of the login shell, even straight
after bootup; login shells are invoked via a program like login(1).
Plus, there is no guarantee the init process actually has PID 1.

This invalidates all use of login_sh that couldn't be replaced by
checks for the login_shell option, so this commit does just that.

src/cmd/ksh93/include/shell.h:
- Remove login_sh flag.

src/cmd/ksh93/sh/init.c:
- If a login shell was detected, just set the login_shell option.
- Remove obsolete check for #! setuid scripts. This was meant to
  guard against a symlink called '-i' to a setuid script with a
  hashbang path, which used to give users a root shell. All modern
  Unixes ignore the setuid bit when they detect a hashbang path.

src/cmd/ksh93/SHOPT.sh:
- By default, let's require the -p/--privileged invocation option
  for the setuid/setgid bit on the shell binary to be respected,
  for all user IDs (>= 0). This is what bash and mksh do, and
  it seems sensible. (See init.c 1475-1483)

src/cmd/ksh93/edit/history.c

@@ -194,8 +194,7 @@ static void hist_touch(void *handle)
 /*
  * open the history file
  * if HISTNAME is not given and userid==0 then no history file.
- * if login_sh and HISTFILE is longer than HIST_MAX bytes then it is
- * cleaned up.
+ * if HISTFILE is longer than HIST_MAX bytes then it is cleaned up.
  * hist_open() returns 1, if history file is open
  */
 int  sh_histinit(void)