check that we have sufficient blocks available in the allocator before
we start a multi-frame transfer to ensure we don't send a corrupt
message by sending a subset of the frames. This can prevent a bus
overload condition where the sender retries on a failed transfer, but
the failed transfer consumes bus traffic and reduces the number of
blocks available for the next transfer

lcov 2.x, unlike lcov 1.x, gives errors like `geninfo: ERROR: mismatched
end line for _ZN12CRC_CRC_Test8TestBodyEv at
/home/runner/work/libcanard/libcanard/tests/test_crc.cpp:42: 42 -> 55`
for source files which contain gtest tests.

These files are removed from the coverage information in a later step,
but lcov now fails to create the initial coverage info because of them.

Fix this problem (and thus CI) by excluding the files from the initial
coverage info instead of removing them later. Note that the exclude
order is relevant to ensure all excludes trigger, avoiding errors that
some are unused. Verified that identical coverage stats are generated
for 2.x as compared to 1.x.

The Subscriber and Client subclasses need semaphores to protect their
static branch lists. The Subscriber previously had one, but it didn't do
anything because it wasn't static, and Client never had one.

Fix the issue by making the HandlerList semaphore protected and re-using
it.

Co-authored-by: Andrew Tridgell <[email protected]>

The handler subclass must not exist in the handler list until after the
superclass constructor runs, and must be removed before the superclass
destructor runs. Outside of this time, the handler object identity is
the superclass, so a call to the handle_message method will cause a pure
virtual error and crash the program.

Fix the issue by adding/removing the object in the handler list in the
subclass when the vtable is set up right and the handle_message can
safely be called. Also wait until the branch list is set up so that the
handle_message method can actually find the object.

Co-authored-by: Andrew Tridgell <[email protected]>

Mitigates flash impact of semaphore changes.

fixes dronecan#79

thanks to  Mykhaylo Shcherbak

use a single linked list for message reception, not a list of lists

this allows for the creation of new HandlerList handlers at runtime