modbus: rename internal designations and do few minor fixups

Prefix internal functions and structs with modbus_.
Use unit_id consistently instead of node_addr.
Fix mbm_ remainder and rename to mbc_.
Rename struct modbus_frame to modbus_adu since
ADU is closer to what the structure represents.

Let the compiler/linker do the job and
remove ifdef around mbc_validate_fc03fp_response().

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>