'.': disable ksh function lookup in POSIX mode

POSIXly, '.' loads only files, not functions.

This only applies to '.', not 'source' (which is not in POSIX).

src/cmd/ksh93/bltins/misc.c: b_source():
- For ksh function lookup, add an additional check that we're not
  in POSIX mode and running the '.' (SYSDOT) builtin.

src/cmd/ksh93/tests/path.sh

@@ -903,5 +903,22 @@ got=$?
 [[ $exp == $got ]] || err_exit "Test 7E: exit status or error message for exec'd command with long name wrong" \
 	"(expected $exp, got $got)"
 
+# ======
+
+if	[[ -o ?posix ]]
+then	(
+		PATH=/dev/null
+		command set --posix
+		function dottest { :; }
+		. dottest
+	) 2>/dev/null && err_exit "'.' in POSIX mode finds ksh function"
+	(
+		PATH=/dev/null
+		command set --posix
+		function dottest { :; }
+		source dottest
+	) 2>/dev/null || err_exit "'source' in POSIX mode does not find ksh function"
+fi
+
 # ======
 exit $((Errors<125?Errors:125))