The new API now covers the entire character driver protocol, while
hiding all the message details. It should therefore be used by all
new character drivers. All existing drivers that already made use of
libchardriver have been changed to use the new API.
As one of the most important API changes, support for scatter and
gather transfers has been removed, as several key drivers already
did not support this, and it could be supported at the safecopy
level instead (for a future readv/writev).
Additional changes include:
- respond to block device open requests to avoid hanging VFS threads;
- add support for sef_cancel.
