Add wl_resource_init and use it in libwayland implementations of data sharing and SHM

This commit adds a wl_resource_init function for initializing wl_resource
structures similar to wl_client_add_object.

From this commit forward, wl_resource structures should not be initialized
manually, but should use wl_resource_init.  In the event of a change to the
wl_resource structure, this allows us to protect against regressions by filling
in added fields with reasonable defaults.  In this way, while changing
wl_object or wl_resource still constitutes an ABI break, compositors following
this rule will only need to be recompiled in order to properly link against the
new version.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>