`UsercallEvent::Start` was being sent in `fn handle_usercall`,
which is too late. It needs to be sent before we receive the next
usercall from the enclave so we can maintain the invariant that
"we only need to keep track of cancels received before the actual
usercall if the read position has not moved past the write position
when cancel was received."

393: [PLAT-896] Verify fifo descriptor pointers r=jethrogb a=raoulstrackx

Some checks are missing when allocating and processing `FifoDescriptor`

Co-authored-by: Raoul Strackx <raoul.strackx@fortanix.com>

398: [PLAT-838] Refactor `ipc_queue` backport r=jethrogb a=raoulstrackx

Backports #396

Co-authored-by: Raoul Strackx <raoul.strackx@fortanix.com>

400: Rust nightly compatibility r=jethrogb a=raoulstrackx

The `mz/async-usercalls` is a rather old branch that hasn't been kept up to date with the latest Rust nightly compilers. This PR corrects this situation by backporting many of the changes already on master.

Co-authored-by: Raoul Strackx <raoul.strackx@fortanix.com>

…n issue #433

432: Enable `enclave_loader()` fallback to libsgx_enclave_common.so.1 r=Taowyoo a=raoulstrackx

The `aesmd` service depends on a `dcap_quoteprov.so` library (the DCAP provider) to aid in DCAP attestations. For it to work correctly, it may need to (recursively) request a DCAP attestation itself. For that a `dcap-ql::enclave_loader` call is required. Unfortunately, when this happens as part of the `aesmd` service, `Dl::this()` returns `/opt/intel/sgx-aesm-service/aesm/aesm_service`. This leads to an error as the expected symbols can't be located. This PR adds a fallback to the `libsgx_enclave_common.so.1` library (or `sgx_enclave_common.dll` on windows).

Co-authored-by: Raoul Strackx <raoul.strackx@fortanix.com>
Co-authored-by: Yuxiang Cao <yuxiang.cao@fortanix.com>