cpuminer: Fix off-by-one issues in nonce handling.
cpuminer: Fix off-by-one issues in nonce handling.

During 32-bit nonce iteration, if a block solution wasn't found, the
iterator variable would overflow back to 0, creating an infinite loop,
thus continuing the puzzle search without ever updating the extra
nonce field.  This bug has never been triggered in practice because
the code in question has only ever been used with difficulties where
a solution exists within the regular nonce space.

The extra nonce iteration logic itself was also imperfect in that it
wouldn't test a value of exactly 2^64 - 1.

The behavior we actually want is to loop through the entire unsigned
integer space for both the regular and extra nonces, and for this
process to continue forever until a solution is found.  Note that
periodic updates to the block header timestamp during iteration ensure
that unique hashes are generated for subsequent generations of the
same nonce values.

jrick/dcrd,decred/dcrd,decred/dcrd,jrick/dcrd