improve calc error code system

Added help/errorcodes rule to the top level Makefile.

Added E_USERMAX symbol (== 32767) to indicate the maximum value
allowed for user error codes.

Improve help/error.  Added text about error code ranges and
range symbols.

Makefile

@@ -325,7 +325,8 @@ STATIC_FIRST_TARGETS= ${LICENSE} .static
 # late targets - things needed after the main build phase is complete
 #
 LATE_TARGETS= calc.1 calc.usage \
-	      cal/.all help/.all help/builtin cscript/.all
+	      cal/.all help/.all help/builtin help/errorcodes \
+	      cscript/.all
 
 # calc tools - tools used by the maintainers of the calc source base
 #
@@ -2465,6 +2466,15 @@ help/builtin: help/Makefile \
 	${V} echo '=-=-=-=-= Back to the main Makefile for $@ rule =-=-=-=-='
 	${V} echo '=-=-=-=-= ${MAKE_FILE} end of $@ rule =-=-=-=-='
 
+help/errorcodes: help/Makefile \
+	      calcerr.h help/errorcodes.hdr help/errorcodes.sed
+	${V} echo '=-=-=-=-= ${MAKE_FILE} start of $@ rule =-=-=-=-='
+	${V} echo '=-=-=-=-= Invoking builtin rule for help =-=-=-=-='
+	${RM} -f $@
+	cd help; ${MAKE} -f Makefile errorcodes
+	${V} echo '=-=-=-=-= Back to the main Makefile for $@ rule =-=-=-=-='
+	${V} echo '=-=-=-=-= ${MAKE_FILE} end of $@ rule =-=-=-=-='
+
 cscript/.all: ${CSCRIPT_TARGETS}
 
 ${CSCRIPT_TARGETS}: cscript/Makefile