libmodbus Fix portable build of libmodbus (upstream)

Upstream the work done for NUT fork of libmodbus, without the RTU USB pieces at the moment - see https://github.com/networkupstools/libmodbus/pull/4 for the equivalent PR there

This subset of those changes focuses more strictly on passing builds without warnings, due to regressions allegedly due to upstream v3.1.10->v3.1.11 bump, so these fixes could be up-streamed separately:

changes in rtu_usb branch not as portable as they should be (printing of int types)
casting of setsockopt() pointer to data
use of inet_ntop()/inet_pton() absent on Windows (at least MINGW on Linux cross-build environments, MSYS2 does not seem to suffer this one) - configure detection and fallback implementation code imported from NUT
ip.h needs in.h explicitly on some platforms

Enhancements:

allow to ./configure --enable-Werror in test builds, to fail the build upon any compiler warnings (not enabled by default), allowing to easily iterate on the code fixes
enhance some debug printouts, add end-of-lines and and console flushing before/after certain messages

There is a separate stack of commits focused on passing make check as well -- the unit-test changes in https://github.com/networkupstools/libmodbus/pull/3, but those are a bit more questionable even to myself (e.g. should flushing be done between tests and/or if some fail? how much should we rely on tight timings in "7ms > 5ms" tests where OS clock for context switches may be not so precise? etc.)

NOTE: I've posted the form for CLA bot some weeks ago, did not see any master-branch changes about that though. Posted another today, just in case.