Enable irqs in kbd/clock calls that caller might "spin" on.

Some old programs will spin on a clock/keyboard call with irqs
disabled.  They assume the BIOS will enable irqs and allow key events
and clock events to occur.

So, enable irqs in those functions that a caller might "spin" on.
diff --git a/src/clock.c b/src/clock.c
index 5a30e35..9afa71d 100644
--- a/src/clock.c
+++ b/src/clock.c
@@ -226,6 +226,7 @@
 static void
 handle_1a00(struct bregs *regs)
 {
+    yield();
     u32 ticks = GET_BDA(timer_counter);
     regs->cx = ticks >> 16;
     regs->dx = ticks;
diff --git a/src/kbd.c b/src/kbd.c
index 6f3ae15..36c89fc 100644
--- a/src/kbd.c
+++ b/src/kbd.c
@@ -73,6 +73,7 @@
 static void
 dequeue_key(struct bregs *regs, int incr, int extended)
 {
+    yield();
     u16 buffer_head;
     u16 buffer_tail;
     for (;;) {
@@ -126,6 +127,7 @@
 static void
 handle_1602(struct bregs *regs)
 {
+    yield();
     regs->al = GET_BDA(kbd_flag0);
 }
 
@@ -184,6 +186,7 @@
 static void
 handle_1612(struct bregs *regs)
 {
+    yield();
     regs->al = GET_BDA(kbd_flag0);
     regs->ah = ((GET_BDA(kbd_flag1) & ~(KF2_RCTRL|KF2_RALT))
                 | (GET_BDA(kbd_flag2) & (KF2_RCTRL|KF2_RALT)));