[fdio] Always provide one handle for PA_FDIO_REMOTE

Previously, converting an zxrio_t into handles could create one or two
handles. After this CL, we always produce exactly one handle.

We obtain the second handle in __libc_extensions_init when we process
the handles from the startup message.

Test: devfs.fdio_tests
Change-Id: Ic980488a7a1f7ad7432d4fad95d03b0ea3d44e46

[devmgr] Implement Describe for devfs

Test: devfs.test_fidl_basic
Change-Id: I18cf472971b2fd3d97dc33c4bf4fcee3386b90bd