vapi: implement vapi_wait() for reads

Type: improvement

The function vapi_wait() is intended to allow a caller to block while
waiting until the API queue can be read/written. It was a stub that
returned VAPI_ENOTSUP. Add code which implements the wait on being able
to read an incoming message.

Had to touch a few other things in vapi.h to make checkstyle.sh happy
after changing the prototype of vapi_wait().

Signed-off-by: Matthew Smith <mgsmith@netgate.com>
Change-Id: Ida80c1a1d34fe297ab23268087be65ea53ad7040