Looks like sys_sysinfo has not been touched in years.  Among other
things, it uses a global cli() for protection; I switched it to an
existing rwlock.  I also pulled it out of info.c and stuck it in timer.c
(I choose timer.c because it shares dependencies there already).

The details:

        - move sys_sysinfo to kernel/timer.c from kernel/info.c:
          why one small syscall got its own file is beyond me.

        - delete kernel/info.c

        - stop the global cli!  now grab a read_lock on xtime_lock.
          this is safe as we moved the write_unlock on xtime_lock
          down one line to cover the calculating of avenrun.

        - trivial code cleanup