Initialize a connection structure. Always check the return value even if
passing in a pre-allocated structure. Some other initialization may have
failed.
*
PARAMETERS
[in] gearman Structure previously initialized with gearman_create() or
gearman_clone().
[in] con Caller allocated structure, or NULL to allocate one.
RETURN VALUE
On success, a pointer to the (possibly allocated) structure. On
failure this will be NULL.