#147 fix warning: ISO C++17 does not allow ‘register’ storage class specifier

[Nick Briggs]

Attached patch fixes all the warnings like:

packet_protocol.cpp: In function ‘void scan_null_header(scan_t)’:
[cc] src/c/packet_protocol.cpp:134:33: warning: ISO C++17 does not allow ‘register’ storage class specifier [-Wregister]
[cc] 134 | register null_header_t nl = (null_header_t *)(scan->buf + scan->offset);

by removing all register attributes (though it doesn't change the comments that mention using register attributes for optimizing speed)