Fixed binary mode for RETR

The original implementation is based on vGetContents which does not work
if the binary file contain byte sequence that could not be decoded into
[Char]. The new implementation is based on vGetBuf which works for
binary data.
Removed rtransmitString

I implemented genericTransmitString based on genericTransmitHandle,
hence could remove the usage of rtransmitString.
Fixed binary issue in cmd_stor

cmd_stor has similar issue with cmd_retr.