Prepare for External mode support
Complete rewrite of External mode connectivity

As it turns out, some assumptions i made for PIL mode (for example,
target reads from receive buffer completely before sending to host) are
not applicable for External mode. Therefore, i had to implement a full
FIFO buffer with semaphores to make External mode work. This
implementation is not yet completely ready, but works.
Add demo model for External mode
Fix rtiostream buffers indexing

Ah, that's why it segfaulted!
Update demo model
Update rtiostreamtest infrastructure to support MLink

See comments in rtiostreamtest_mdaq.m
Update analog demo models