Replace kern_{system,unix}_time() system calls with kern_time_{get,set}()

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>
Add kern_system_info() system call

Add a system call for retrieving system information. Currently we have
one bit of information that can be retrieved, the system page size.

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>