add errsym builtin function

NOTE: errstr was renamed to errsym.

Added errsym builtin function.  The errsym(errnum | "E_STRING")
builtin, , when given a valid integer errnum that corresponds to a
calc error condition, will return an E_STRING string, AND when given
a valid E_STRING string that is associated with a calc error
condition, will return errnum integer that corresponds to a calc
error condition.

Supplying a non-integer numeric errnum code to error(), errno(),
strerror(), or errsym() will result in an error.

help/iserror

@@ -25,7 +25,7 @@ LINK LIBRARY
     none
 
 SEE ALSO
-    errcount, errmax, errno, errorcodes, errstr, newerror, stoponerror, strerror
+    errcount, errmax, errno, errorcodes, errsym, newerror, stoponerror, strerror
 
     isassoc, isatty, isblk, isconfig, isdefined, iseven, isfile,
     ishash, isident, isint, islist, ismat, ismult, isnull, isnum, isobj,