Prevent setting -blockmaxsize and -excessiveblocksize <= 0

Summary: This makes the init check consistent with the RPC. The GetArg() function returns an int which will then be casted to uint and might give unexpected results.

Test Plan:

./test/functional/test_runner.py abc-cmdline

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9744