PROTON-697: part 1, server support and documentation changes for certificate stores/dbs

This patch provides server-side support, and PN_SSL_ANONYMOUS_PEER
connections.

The most important changes for review are the documentation changes to
ssl.h regarding certificates.  Windows first class citizens are
registry stores and pkcs#12 files, both which resemble NSS databases
and Java keystores.

Re-purposing the "private_key_file" parameter to alternatively denote a
certificate name in a database/store works in this case for Windows
and presumably for Java keystores and NSS databases.  Hopefully for
any other SSL/TLS implementations we may encounter too.

I am unable to find a convention for identifying Windows system (or
registry) certificate stores.  The command line option for the common
case is "-ss name_of_store_or_its_alias", and I have adopted "foo" or
"c:\path\to\foo" to mean a file based store and "ss:bar" to mean a
user system store named "bar" and "lmss:bar" for the less common Local
Machine system store designation.  I'm not sure of the best place to
document the platform specific naming conventions.

At the moment you may have any system store you like as long as it is
"ss:root".  That limitation should be lifted in a few days.