Zero is a valid route metric and distinct from -1, which means unspecified.
Fix reader and writer.

Fixes: e374923bbe4a9f608644756f749b9bae9aa5f349

Users are probably more familiar with iproute2 route option names than
kernel ones.

Fixes: 54e58eb96bbfcd26d31ddba2e98ff2c59335a02a

Add API to re-read the keyfile after writing it.
Usually, we would expect that whenever we serialize
something to disk, it can be read back exactly the same.

That is however not true for certificates, where we mangle
path and blobs while writing to file.

Anyway, extend the write-API to re-read what we just wrote.

The tests got extended to assert that whatever we write can
be read back the same.

Later, we want to reinject the reread connection into the settings
plugin again.

Catches previously fixed memleak in read_array_of_uint()

Keep the include paths clean and separate. We use directories to group source
files together. That makes sense (I guess), but then we should use this
grouping also when including files. Thus require to #include files with their
path relative to "src/".

Also, we build various artifacts from the "src/" tree. Instead of having
individual CFLAGS for each artifact in Makefile.am, the CFLAGS should be
unified. Previously, the CFLAGS for each artifact differ and are inconsistent
in which paths they add to the search path. Fix the inconsistency by just
don't add the paths at all.