Fix seeder compilation warnings

Summary:
Several of these on MacOSX at least

CXX seeder/libbitcoin_seeder_a-protocol.o
In file included from seeder/protocol.cpp:8:
./seeder/util.h:51:23: warning: passing an object of reference type to 'va_start' has undefined behavior [-Wvarargs]

va_start(arg_ptr, format);
                  ^

Everywhere actually passes a C string not a std::string so it seemed simplest to just switch to that.

Test Plan: make check; run binary

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D585