[fidl] Generate symbol for discoverable name of interfaces in C bindings

This teaches the C bindings generator to emit a preprocessor symbol
containing the discoverable name for interfaces with the 'Discoverable'
attribute and uses that instead of hand coded string constants in
places that use the name of interfaces. This is slightly less
error-prone and centralizes the name mangling logic.

Test: Build and boot anything that creates a process
Change-Id: Idafada21b8c2ef540f18e656fb9637ae5dcb4fde

[async] Change async_t -> async_dispatcher_t

Change name of dispatcher type from async_t to async_dispatcher_t
Keep typedef of async_t for compatibility

Change async_{get,set}_default() to async_{get,set}_default_dispatcher()
Again, keep compatible apis until other layers have been modified to use
new api.

Bug:ZX-2337
Test:CQ
Change-Id: If0b5d061fd114d60e23078dbb5f2a6ed9ed6f6cc

[svchost] Introduce svchost

The svchost process will be able to host Zircon-level services, such as
process-launcher. Eventually, svchost will be started by devmgr and
exposed to processes launched by devmgr as /svc.

Change-Id: Id5835cf6c546119d1d533144d6443283070a77e1