boards/native_posix: Unwrap main loop to expose external event API

Some applications of native_posix (libfuzzer in particular) want to
drive the native_posix environment from "outside" as a caller.

Unwrap the hwm_main_loop() utility into a callable hwm_one_event()
function instead, and move the loop into the main() function instead.
Likewise split out the initialization path into a API-visible
posix_init() function.

Finally adds a posix_exec_for() entry point which iterates over the
event interface for a given timeout.

Signed-off-by: Andy Ross <andyross@google.com>