[GDB] Handle kill and detach packets.

This commit also includes a test to ensure that single stepping works,
since continue, kill, detach, and single step all share code.

src/tests/gdbstub_test.gdb

@@ -71,6 +71,12 @@ define gpxe_test_mem_write
 	gpxe_assert ({char}($esp)) (char)0x99 "gpxe_test_mem_write char"
 end
 
+define gpxe_test_step
+	c
+	si
+	gpxe_assert ({char}($eip-1)) (char)0x90 "gpxe_test_step" # nop = 0x90
+end
+
 gpxe_load_symbols
 gpxe_connect
 gpxe_start_tests
@@ -78,3 +84,4 @@ gpxe_test_regs_read
 gpxe_test_regs_write
 gpxe_test_mem_read
 gpxe_test_mem_write
+gpxe_test_step