vlapi: SendCommandReadBulk() shift baudrate parameter

Shift baudrate parameter next to Port parameter, because
the two belong together and resolve an ambiguity problem
in the next patch
vlapi: add documentation to ReadBulk functions

Add some api documentation
vlapi: SendCommandReadBulk() implement optional longer timeout for first char

When downloading the binary log from the logger it is necessary to
wait a longer period after the first ACK is sent to the logger until
the first char is received.
This is because the logger starts calculating file security before the
first byte is sent. This may take several minutes.
The additional parameter needs to be implemented in ReadBulk() and
subsequently also in SendCommandReadBulk()
vlapi: readdir() change return type to return read data length

This is necessary since we need to pass the length of the read data to
conv_dir() in a later patch.
vlapi: download signature with standard IO rate

Testing has shown, that a download with bulkrate is not possible for
downloading the file signature.
Standard rate of 9600 baud has to be used at all times.
The download of the main log data however supports bulk rate.