Attempt to (incompletely) document EVP_aes_*().

When EVP_des_cbc() was suggested, suggest EVP_aes_256_cbc() instead.

Remove mention of EVP_des_ede3_cbc() being the algorithm of choice for S/MIME.

Don't mention US-export limited RC2 algorithms, you'd better not know about
them.
Features introduce in OpenSSL 0.9.8 do not deserve their own section.
Instead, fold their description in the main documentation, and update the
history section to mention them as well.
Fewer lies, also do not document DES_3cbc_encrypt anymore.
Remove empty sections and references to des_modes(7) which we don't have.
Stop including standard headers via cryptlib.h - pull in the headers that
are needed in the source files that actually require them.

ok beck@ miod@
Try and fix the horrible coding style of the example code snippets.
Fix examples description, and use less ambiguous wording.