tree-wide: use ERFKILL instead of ESHUTDOWN for "unit masked"

tree-wide: use ERFKILL instead of ESHUTDOWN for "unit masked"

If the error code ever leaks (we print the strerror error instead of providing
our own), the message for ESHUTDOWN is "Cannot send after transport endpoint
shutdown", which can be misleading. In particular it suggest that some
mishandling of the dbus connection occured. Let's change that to ERFKILL which
has the advantage that a) it sounds implausible as actual error, b) has the
connotation of disabling something manually.