lib: posix: eventfd: fix read/poll after multiple writes

So far semaphore was used with possible values in range 0 to
UINT32_MAX. Each write resulted in semaphore increment. As an example
after two writes and single read eventfd counter was correctly zeroed,
but semaphore counter was not. This means that poll() signalled at this
stage POLLIN (semaphore counter was > 0), but it clearly should
not (eventfd counter == 0). Blocking version of read() was also
returning immediately, returning 0 as previous eventfd counter.

Change read_sem to be a binary semaphore, which counter represents
eventfd counter being zero (when semaphore counter == 0) or
non-zero (when semaphore counter == 1). Try to take the semaphore in
eventfd read() and decrement eventfd counter when semaphore was ready.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>