[syscalls] Remove read_status from zx_channel_call

This parameter no longer has a purpose. Also, remove ZX_ERR_CALL_FAILED.
This status code no longer has a purpose.

Test: No behavior change
Change-Id: I71945ea9d1d6d591a20476106df1e598dfaf97b1

[utest] Use ZX_TIME_INFINITE over UINT64_MAX in thread-functions.cpp

We're planning to change zx_time_t from an unsigned type to a signed
type. This change makes thread-functions.cpp agnostic of zx_time_t's
signedness.

ZX-2100 #comment progress

Change-Id: Ifd7b17d8de3b985c5b01bc726180da974cdc2d51

[syscalls] Remove count arguments from port_queue and port_wait

This argument is not used and always must be one. I tried implementing
some reasonable behavior for this parameter on port_queue, but it wasn't
clear what to do when we encounter an error during the queue operation
because there isn't an out parameter for how many we succeeded in
queuing.

Rather than having a half-baked feature, we should just remove the
parameter and add another function that does vectorized versions of
these operations if we need that in the future.

Change-Id: Ifffcd6d705442cc830912a67c8117ffe5ec8295d

[test] Rename test-threads to thread-functions.

The old naming meant that there were two threads.h and threads.cpp files
in the threads test directory tree. The new naming makes it more clear
that the file contains the functions used to run the backround threads
for the tests.

Change-Id: I320fdb73fd7ed5978a3279a4715ef083ecab185c