Add GETDIM @-code to get the current ANSI terminal dimensions (rows and cols)

This code will do nothing if there's no user logged-in or their terminal is
not ANSI or their terminal dimensions (rows and cols) are set to specific
values (not auto-detected). This code does the same thing as calling JS
console.getdimensions(). Since it waits (up to 5 seconds) for a CPR response
from the terminal, this does potentially slow down output, so caveat emptor.

This could be used by a sysop to address issue #529.

src/sbbs3/atcodes.cpp

@@ -731,6 +731,11 @@ const char* sbbs_t::atcode(char* sp, char* str, size_t maxlen, int* pmode, bool
 		return(nulstr);
 	}
 
+	if(strcmp(sp, "GETDIM") == 0) {
+		ansi_getlines();
+		return nulstr;
+	}
+
 	if(strcmp(sp, "GETKEY") == 0) {
 		getkey();
 		return(nulstr);