* Fix uart regressions & bugs.
Using `uart.on()` with a search character was broken in that it did
not invoke the callback on a full UART buffer as documented. Logic reworked
to match docs again.
Fixed memory leak on `task_post()` failure (eep!).
Improved logic to attempt to coalesce input bytes to reduce the number of
`task_post()` slots used up by the platform uart.
Finally, added a semaphore to prevent the platform uart from overrunning
the `task_post()` slots all the time on high baud rates (e.g. 1mbit).
With the semaphore in there, the LVM RTOS task gets a chance to actually
process the received data and free up a `task_post()` slot or two.
The above mentioned read coalescing then allows the platform uart to
immediately catch up.
Also added an error log message if the `task_post()` actually does fail.
* Don't cache the uart delims.
Doing so makes reconfiguring those settings from within the callback not
take effect until the currently buffered bytes have been processed.
b0558d5bbe