Improve integer-type hygiene in bignum code.

Improve integer-type hygiene in bignum code.

In many places I was using an 'unsigned int', or an implicit int by
virtue of writing an undecorated integer literal, where what was
really wanted was a BignumInt. In particular, this substitution breaks
in any situation where BignumInt is _larger_ than unsigned - which it
is shortly about to be.

(cherry picked from commit e28b35b0a39de28fa2f71aa78071d1ad62deaceb)

Conflicts:
	sshbn.c
	sshccp.c

Cherry-picker's notes: the conflicts were because the original commit
also modified new code (sshccp.c for dedicated Poly1305 arithmetic
routines, sshbn.c for a few bignum functions introduced on trunk for
various pieces of new crypto) which doesn't exist on this branch.