Clean-up: clang-tidy autofixes to eventfd and file-descriptor

Clean-up: clang-tidy autofixes to eventfd and file-descriptor

eventfd must be marked explicit to prevent erroneous implicit
conversions.

A destructor is provided since other special member functions are
defined (see cppcoreguidelines-special-member-functions).

file_descriptor's constructor is replaced by a trivial constructor.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I1623a2f8db0d613201a534293f5e1476b779283c