kernel terminal should initialize fg and bg variables before calling TUNABLE_INT_FETCH

We have two ways to check if kenv variable exists - either we check return
value from TUNABLE_INT_FETCH, or we pre-initialize the variable and check
if this value did change. In terminal_init() it is more convinient to
use pre-initialized variables.

Problem was revealed by older loader.efi, which did not set teken.* variables.

Reported by:	tuexen

sys/kern/subr_terminal.c

@@ -175,6 +175,7 @@ terminal_init(struct terminal *tm)
 
 	teken_init(&tm->tm_emulator, &terminal_drawmethods, tm);
 
+	fg = bg = -1;
 	TUNABLE_INT_FETCH("teken.fg_color", &fg);
 	TUNABLE_INT_FETCH("teken.bg_color", &bg);