python: Add set_trap_handler

This provides access to seccomp_decode_sigsys and sigaction(SIGSYS, ...)

You can use it like this:

import os
import seccomp
seccomp.set_trap_handler(seccomp.print_trapped_syscall)
f = seccomp.SyscallFilter(seccomp.ALLOW)
f.add_rule(seccomp.TRAP, 'getgid')
f.load()
os.getgid()
print "Unreachable"

This prints:

Dying due to seccomp-blocked syscall getgid(0x0, 0x0, 0x4, 0x0, 0x0, 0x3AD8F13240)

(Note: those arguments are bogus.  A considerably fancier future version
 could know the syscall signatures and the mapping from raw to logical
 arguments to allow a nicer message like "... getgid()".  In the mean time,
 this makes debugging much easier.)

Signed-off-by: Andy Lutomirski <luto@amacapital.net>