More robust control sequence parameter handling.

More robust control sequence parameter handling.

Parameters are now accumulated in unsigned integers and carefully checked
for overflow (which is turned into saturation).  Things that consume them
now have explicit range checks (again, saturating) to ensure that their
inputs are sane.  This should make it much harder to cause overflow by
supplying ludicrously large numbers.

Fixes two bugs found with the help of afl-fuzz.  One of them may be
exploitable and is CVE-2015-5309.