Add opencl_get_vector_width(device_id, size) function to common-opencl.[ch]
and have all vector-capable formats use it.
New OpenCL LWS/GWS enumeration scheme:
1. Enumerate GWS using *LWS == NULL.
2. Enumerate LWS using the now known GWS.

Affected formats: NTLMv2, RAR, krb5pa-md5, krb5pa-sha1, wpapsk, rakp,
mysql-sha1 and office formats.