const can be specified for a function argument more than once

[Daniel-Cortez]

Issue description:

While working on an implementation of named-only function arguments (#719), I found an oversight in function declargs().

compiler/source/compiler/sc1.c

Lines 4278 to 4282 in 81b4a02

	case tCONST:
	if (ident!=iVARIABLE || numtags>0 || fpragma)
	error(1,sc_tokens[tSYMBOL-tFIRST],sc_tokens[tCONST-tFIRST]);
	fconst=TRUE;
	break;

There's no check if const was already specified, although handlers for other specifiers (&, Tag:, ...) have this kind of check.
As a result, the user can specify const any number of times

Func(const const const const arg) {}

and the compiler would think this is okay, although it clearly isn't.

Minimal complete verifiable example (MCVE):

See above.

Workspace Information:

• Compiler version: 3.10.10
• Command line arguments provided (or sampctl version): Not relevant.
• Operating System: Not relevant.