bus: allow reading empty arrays
bus: fix bus_message_read_strv

message_read_strv_extend returns 0 on success
bus: fix bus_print_property with strv
bus: rename sd_bus_get_property_{trivial,basic}
bus: mark sd_bus_message_at_end public
analyze: split out bus util functions

They will soon be used in systemctl
systemctl: port to libsystemd-bus

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>