Merge dtksh patches from one of the CDE developers (#85)

This merges some fixes to support building dtksh with -DBUILD_DTKSH.
These patches were sent through private email from the CDE developer
Chase. The reason these patches were submitted is because Chase wishes
to include ksh in CDE as an up-to-date git submodule. Quote from Chase:
"... my priority is to get your new version into our code as a git
 submodule, and do it quickly before our code bases differ too widely."

Link to CDE project for anyone interested:
https://sourceforge.net/projects/cdesktopenv/

Although the patches were privately discussed, there are some public
emails on the CDE mailing list (links shortened due to long URLs):
ksh-chaos thread:   https://bit.ly/3hjJ83b
dtksh alias thread: https://bit.ly/3hkzKfJ

The main fix is for suid_exec, which is now told that /usr/dt is a
valid directory to run from via preprocessor flags. A patch for
Shift-JIS was also submitted, but it isn't in this commit because it
isn't an effective fix for the existing Shift-JIS bugs. I will be
giving that patch some more testing.

From: Chase <nicetrynsa@protonmail.ch>
Co-authored by: Johnothan King <johnothanking@protonmail.com>

src/cmd/ksh93/sh/suid_exec.c

@@ -62,7 +62,11 @@
 #define FDVERIFY	12	/* used to validate /tmp process */
 #undef BLKSIZE 
 #define BLKSIZE		sizeof(char*)*1024
+#if BUILD_DTKSH
+#define THISPROG	SUIDEXECPATH
+#else
 #define THISPROG	"/etc/suid_exec"
+#endif
 #define DEFSHELL	"/bin/sh"
 
 static void error_exit(const char*);
@@ -235,6 +239,9 @@ exec:
 	/* only use SHELL if file is in trusted directory and ends in sh */
 	shell = getenv("SHELL");
 	if(shell == 0 || !endsh(shell) || (
+#ifdef BUILD_DTKSH
+		!in_dir(CDE_INSTALLATION_TOP"/bin",shell) &&
+#endif
 		!in_dir("/bin",shell) &&
 		!in_dir("/usr/bin",shell) &&
 		!in_dir("/usr/lbin",shell) &&