Avoid dependencies but explicitly link the static library where it is
used.
This also fixes that we linked libnm-log-core into
libnm-settings-plugin-ifcfg-rh.so, which duplicated the symbols
while it should used them from NetworkManager.
This helper code is already used by several of our unit tests.
Compile it as a separate library.
Previously, the source code lingered unmotivated under "shared/",
which is confusing.
