libweston: Introduce an internal linux sync file API

Introduce an internal API for dealing with linux sync files,
and use it in the codebase to replace ad-hoc sync file management.

The linux_sync_file_is_valid function is not currently used, but will be
utilized in upcoming commits to implement the
zwp_linux_explicit_synchronization_unstable_v1 protocol.

Changes in v5:
  - Meson support.

Changes in v3:
  - Use parameter name in function documentation.
  - Move kernel UAPI to separate header file.

Changes in v2:
  - Add function documentation
  - Remove linux_sync_file_wait()

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>