Fix $PWD breakage on fork; cd; exec (rhbz#1168611)

This applies the following Red Hat patch:
https://src.fedoraproject.org/rpms/ksh/blob/642af4d6/f/ksh-20120801-cdfork.patch

The associated bug report is public, but nearly all info (such as
a reproducer) has been wiped: https://bugzilla.redhat.com/1168611
However, the errata blurb is mildly informative:

"Previously, ksh sometimes incorrectly initialized a variable
holding the path of the working directory. If a program changed the
working directory between forking and ksh execution, then ksh could
contain an incorrect value in the working directory variable. With
this update, initialization of the working directory variable has
been corrected, and ksh now contains the correct value in the
aforementioned situation."

Also, the patch makes a lot of sense on the face of it. It removes
an optimisation in path_pwd() that checks for the directory defined
by e_crondir[] in data/msg.c, which is:

    const char e_crondir[]		= "/usr/spool/cron/atjobs";

Of /usr/spool not existed on any system for decades as it is common
to mount usr as read-only, so all the writable stuff was moved to
/var. So that would never check out. And if 'flag' is nonzero, the
optimizing 'count++' is executed regardless of whether that
directory exists, ensuring that it never gets the real PWD and
defaults to returning ".".

src/cmd/ksh93/sh/path.c:
- Apply patch as described.
- Mark 'flag' variable as NOT_USED to suppress compiler warning.
  Keep it for backwards compat, as some programs that link with
  libshell might use this function (though it's undocumented).

src/cmd/ksh93/include/path.h,
src/cmd/ksh93/data/msg.c:
- Remove now-unused e_crondir[].

src/cmd/ksh93/data/msg.c

@@ -188,7 +188,6 @@ const char e_suidprofile[]	= "/etc/suid_profile";
 #if SHOPT_SYSRC
 const char e_sysrc[]		= "/etc/ksh.kshrc";
 #endif
-const char e_crondir[]		= "/usr/spool/cron/atjobs";
 const char e_prohibited[]	= "login setuid/setgid shells prohibited";
 #ifdef BUILD_DTKSH
    const char e_suidexec[]      = SUIDEXECPATH;

src/cmd/ksh93/include/path.h

@@ -117,7 +117,6 @@ extern const char e_mailmsg[];
 extern const char e_suidprofile[];
 extern const char e_sysprofile[];
 extern const char e_traceprompt[];
-extern const char e_crondir[];
 #if SHOPT_SUID_EXEC
     extern const char	e_suidexec[];
 #endif /* SHOPT_SUID_EXEC */

src/cmd/ksh93/sh/path.c

@@ -236,14 +236,14 @@ static pid_t path_xargs(Shell_t *shp,const char *path, char *argv[],char *const
 /*
  * make sure PWD is set up correctly
  * Return the present working directory
- * Invokes getcwd() if flag==0 and if necessary
+ * Invokes getcwd() if necessary
  * Sets the PWD variable to this value
  */
 char *path_pwd(Shell_t *shp,int flag)
 {
 	register char *cp;
-	register char *dfault = (char*)e_dot;
 	register int count = 0;
+	NOT_USED(flag);
 	if(shp->pwd)
 		return((char*)shp->pwd);
 	while(1) 
@@ -261,11 +261,6 @@ char *path_pwd(Shell_t *shp,int flag)
 				cp = "/";
 				break;
 			case 3:
-				cp = (char*)e_crondir;
-				if(flag) /* skip next case when non-zero flag */
-					++count;
-				break;
-			case 4:
 			{
 				if(cp=getcwd(NIL(char*),0))
 				{  
@@ -276,8 +271,8 @@ char *path_pwd(Shell_t *shp,int flag)
 				}
 				break;
 			}
-			case 5:
-				return(dfault);
+			case 4:
+				return((char*)e_dot);
 		}
 		if(cp && *cp=='/' && test_inode(cp,e_dot))
 			break;