shared: move unit-specific code from bus-util.h to bus-unit-util.h

shared: move unit-specific code from bus-util.h to bus-unit-util.h

Previously we'd have generally useful sd-bus utilities in bust-util.h,
intermixed with code that is specifically for writing clients for PID 1,
wrapping job and unit handling. Let's split the latter out and move it into
bus-unit-util.c, to make the sources a bit short and easier to grok.