Fix corner case bug involving 'eval' and return < 0 (re: b90b8cd)

Reproducer:

    $ foo() { return -1; }
    $ eval foo
    -ksh: eval: not found

ksh tries to run 'eval' as an external command. No such external
command generally exists on $PATH, hence the error message.

Analysis:

Built-ins may have the BLT_EXIT attribute (see data/builtins.c)
which has two functions:
1)  To avoid trimming a built-in's exit status to the least
    significant eight bits (xec.c, lines 1278-1279).
2)  To fall back to an external command by the same name if the
    built-in returns an exit status < 0 (xec.c, lines 1338-1345).
    This of course depends on the exit status not being trimmed.

As of the referenced commit which made 'return' accept a negative
exit status, feature 2 triggers this bug. 'eval' has the BLT_EXIT
attribute, and it needs it beecause of feature 1, but feature 2 is
not desired here.

Feature 2 also unused, and (as far as I can tell) always has been.
No regular built-in falls back to an external command. Some libcmd
built-ins (date, getconf, uname) can fall back to their external
counterparts, but they use the sh_run() interface to do so.

Since feature 2 is unused except for triggering this bug, the
simplest fix is to remove it. This commit does that.

src/cmd/ksh93/include/version.h

@@ -18,7 +18,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.1.0-alpha"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2024-11-17"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2024-11-24"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2024 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh/xec.c

@@ -1335,14 +1335,7 @@ int sh_exec(const Shnode_t *t, int flags)
 					sh.redir0 = 0;
 					if(jmpval)
 						siglongjmp(*sh.jmplist,jmpval);
-					if(sh.exitval >=0)
-						goto setexit;
-					/*
-					 * If a built-in sets sh.exitval < 0 (must be allowed by BLT_EXIT attribute),
-					 * then fall through to TFORK which runs the external command by that name
-					 */
-					np = 0;
-					type=0;
+					goto setexit;
 				}
 				/* check for functions */
 				if(!command && np && nv_isattr(np,NV_FUNCTION))

src/cmd/ksh93/tests/return.sh

@@ -279,5 +279,15 @@ trap - USR1
 unset -f f
 [[ $(<out) == 0 ]] || err_exit "default return status in traps is broken (expected 0, got $(<out))"
 
+# ======
+print 'print "BUG TRIGGERED"' >eval
+chmod +x eval
+PATH=$PWD:$PATH
+foo() { return -1; }
+got=$(eval foo)
+[[ -z $got ]] || err_exit "negative return status from 'eval' triggers external 'eval' command (got $(printf %q "$got"))"
+PATH=${PATH%"$PWD:"}
+unset -f foo
+
 # ======
 exit $((Errors<125?Errors:125))