wmeta: Add support for float and double types
Remove duplicate "const" specifier in declaration

Duplicate "const" specifiers in declarations cause compiler warnings when
building with Clang or GCC 4.7+.
wevent: Allow setting the backend with W_EVENT_BACKEND

The backend used for w_event_t can now be manually specified by defining
W_EVENT_BACKEND to either W_EVENT_BACKEND_EPOLL or W_EVENT_BACKEND_KQUEUE
manully when building. If undefined, the value will be guessed using the
existing logic.
Update queue.h to latest OpenBSD version (1.38)