Fix build errors on Ubuntu 12.04.

  CC    syscalls/prctl.o
syscalls/prctl.c:33:13: error: ‘PR_SET_CHILD_SUBREAPER’ undeclared here (not in a function)
syscalls/prctl.c:33:37: error: ‘PR_GET_CHILD_SUBREAPER’ undeclared here (not in a function)
syscalls/prctl.c:34:23: error: ‘PR_GET_TID_ADDRESS’ undeclared here (not in a function)

Signed-off-by: Vinson Lee <vlee@twitter.com>
Fix buffer overflow in output() when pid slot is not found

In output() we sprintf() the result of find_pid_slot(). We print the pid
slot to the buffer with %u and have space for two digits of pid slot.
find_pid_slot() potentially returns PIDSLOT_NOT_FOUND (-1), which when
printed with %u is 4294967295 - ten digits.

Fix it two ways, use snprintf() - truncated output is better than a
buffer overflow. And allocate more space in the buffer, 32 bytes is a
nice round size, and gives us space for everything.
Compat fixes for DRM