Remove SH_NOLOG vestiges

The '-o nolog' option (which prevented function definitions from being
recorded in the history file) was removed a long time ago, leaving
only a stub for backwards compatibility to stop 'set' from erroring
out if the option is set. But some other vestiges remained.

src/cmd/ksh93/sh/path.c,
src/cmd/ksh93/sh/subshell.c,
src/cmd/ksh93/sh/xec.c:
- Remove a few pointless 'sh_onstate(SH_NOLOG)' statements. As of
  93u+ or earlier, this is never checked for anywhere.

src/cmd/ksh93/sh.1:
- They forgot to remove the 'nolog' option documentation here.
  Specify that it's obsolete and has no effect.

src/cmd/ksh93/data/builtins.c: sh_set[]:
- Be more concise.

src/cmd/ksh93/sh/xec.c

@@ -2919,7 +2919,6 @@ pid_t _sh_fork(Shell_t *shp,register pid_t parent,int flags,int *jobid)
 	shp->login_sh = 0;
 	sh_offoption(SH_LOGIN_SHELL);
 	sh_onstate(SH_FORKED);
-	sh_onstate(SH_NOLOG);
 	if (shp->fn_reset)
 		shp->fn_depth = shp->fn_reset = 0;
 #if SHOPT_ACCT