Add engineering output mode

Similar to scientific mode, engineering mode also displays numbers in
base 10 exponential notation, with the difference that exponents are
always multiples of 3, to facilitate the interpretation in terms of
SI prefixes.

The mode is activated in config thru "engineering" or "eng. For base
and base2, it uses the special value 1000.

file.c

@@ -1048,6 +1048,9 @@ idprintf(FILEID id, char *fmt, int count, VALUE **vals)
 		case 'e':
 			newmode = MODE_EXP;
 			break;
+		case 'n':
+			newmode = MODE_ENG;
+			break;
 		case 'g':
 			newmode = MODE_REAL_AUTO;
 			break;