Let symbols have a location.

Let symbols have a location.
* src/symtab.h, src/symtab.c (symbol_t): Location is a new member.
(getsym): Adjust.
Adjust all callers.
* src/complain.h, src/complain.c (complain_at, fatal_at, warn_at):
Use location_t, not int.
* src/symtab.c (symbol_check_defined): Take advantage of the
location.
* tests/regression.at (Invalid inputs): Adjust.