Change the EVP_somecipher() and EVP_somedigest()
functions to return constant EVP_MD and EVP_CIPHER
pointers.

Update docs.

sure they are available in opensslconf.h, by giving them names starting
with "OPENSSL_" to avoid conflicts with other packages and by making
sure e_os2.h will cover all platform-specific cases together with
opensslconf.h.

I've checked fairly well that nothing breaks with this (apart from
external software that will adapt if they have used something like
NO_KRB5), but I can't guarantee it completely, so a review of this
change would be a good thing.

Add a couple of FAQs.

structure and decides what key to generate (if any). Not currently added to
the PBE algorithm list because it is largely untested.