This change allows the set_mode functions to leave the rig passband
unchanged if required. For the few rigs that do not have explcit
passband width control either current state is read and rewritten or a
"normal" width is chosen e.g. select a normal width when there is a
choice like CW and CW-NARROW.
Various strncpy operations could result in a port pathname that is not a
NULL terminated string as the allowed string length is the same size as
the buffer per the strncpy manual page. This is corrected by assuring
that the allowed length is FILPATHLEN - 1.