[ulib][async] Add support for exception ports

Tested: runtests
[which includes new tests for the new functionality]

Change-Id: Ib8eaf30eb2d9abf3f3ca72a0d77450ea52151149

[zx] Migrate libzx off legacy_unowned<T> and update call-sites.

- Migrate APIs like zx::process::self() from returning a
|const zx::legacy_unowned<T>| to returning |zx::unowned<T>|.
- Update callers of those APIs to use the appropriate dereference
operator to access members of T, or to get a |const T&| to pass to
libzx-wrapped syscalls.
- Remove the zx::legacy_unowned<T> template.

Bug: ZX-2310
TEST=CQ
Change-Id: Idd4267cd0246ec9ab0e6f1392efe7fda45ff169d

[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