index: raise children_n limit to struct 'I' max

index: raise children_n limit to struct 'I' max

This assertion appears to be a sanity check, and one that's perhaps too
low (think giant Maildirs), so raise it to the real limit, which should
be UINT_MAX, given that the index header containing it is
encoded/decoded via 'I' (platform native unsigned int):

  https://docs.python.org/2/library/struct.html

Since Python doesn't appear to provide access to UINT_MAX, add it (and
INT_MAX) to _helpers.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>