bus: make sd_bus_request_name() and sd_bus_release_name() behave more like other calls

Instead of returning an enum of return codes, make them return error
codes like kdbus does internally.

Also, document this behaviour so that clients can stick to it.

(Also rework bus-control.c to always have to functions for dbus1 vs.
kernel implementation of the various calls.)
journal: fail silently in sd_j_sendv() if journal is unavailable

"syslog(3) and sd_journal_print() may largely be used interchangeably
functionality-wise" according to sd_journal_print(3). This socket
should be always available except in rare circumstatances, and we
don't random applications to fail on logging, so let's do what syslog
did. The alternative of forcing all callers to do error handling for
this rare case doesn't really have any benefits, since if they can't
log there isn't much they can do anyway.

https://bugzilla.redhat.com/show_bug.cgi?id=1023041