Unify X.509v3 extension methods

Unify X.509v3 extension methods

Use C99 initializers for all structs (some were forgotten).
Make all the structs static, call them x509v3_ext_* matching NID_*.
Add accessors called x509v3_ext_method_* and use these to implement
X509V3_EXT_get_nid().

This adds consistency and avoids a few contortions like grouping
a few extensions in arrays to save a couple externs.

ok beck jsing