Fix compilation errors in rewrite branch

- Fix undeclared variable 'jws' -> 'jwk' in src/jws.c
- Add missing #include <openssl/ec.h> for EC types
- Fix undeclared variable 'pkey' -> 'key' in jwk_thumbprint
- Add missing 'pad' parameter to base64url_encode_inplace call
- Add const qualifier to jws_builder_write signature
- Add common_name, country, org fields to ACME struct
- Initialize certificate fields in acme_init
- Implement json_streq helper function
- Fix typo: complete_finalize_request -> complete_finalize_order_request

All compilation errors resolved. Only deprecation warnings remain for OpenSSL 3.0 APIs.

Author: claude