Fixed compile error with GNUTLS+NETTLE

In nettle/md5.h, md5_init and md5_update are defined as macros to
nettle_md5_init and nettle_md5_update respectively.  This causes
error when using MD5_params.md5_init and md5_update.  This patch
renames these members as md5_init_func and md5_update_func to
avoid name conflict. For completeness, MD5_params.md5_final was
also renamed as md5_final_func.

The changes in curl_ntlm_core.c is conversion error and fixed by
casting to proper type.