Split X509_NAME_hash(3) out of d2i_X509_NAME(3) and document

Split X509_NAME_hash(3) out of d2i_X509_NAME(3) and document
X509_issuer_name_hash(3), X509_subject_name_hash(3), and the _old variants.

Even though this is only tangentially related to decoding and encoding,
including a single function in d2i_X509_NAME(3) was probably OK,
but let's not bog down that page with six functions that are likely
to become obsolete at some point - even though right now, they are
still being used both internally and by external software.