openssl.cnf tweaks following recent changes to usr.bin/openssl:

- don't define default_bits, allowing the compiled-in default (now 2048
bits) to take priority.

- add commented-out default_md line in case somebody needs an easy way
to change this.

- remove some sample sections which aren't really useful in the default
file (/etc/examples is the place for a more descriptive config, this
file should be barebones).

Help/OK jsing@. OKs on earlier diff (openssl.cnf only) from phessler@ aja@.