New ctrl sets current certificate based on certain criteria. Currently
two options: set the first valid certificate as current and set the
next valid certificate as current. Using these an application can
iterate over all certificates in an SSL_CTX or SSL structure.

PR#3169

This patch, which currently applies successfully against master and
1_0_2, adds the following functions:

SSL_[CTX_]select_current_cert() - set the current certificate without
disturbing the existing structure.

SSL_[CTX_]get0_chain_certs() - get the current certificate's chain.

SSL_[CTX_]clear_chain_certs() - clear the current certificate's chain.

The patch also adds these functions to, and fixes some existing errors
in, SSL_CTX_add1_chain_cert.pod.

This fixes problems in POD list formatting: extra or missing =back
sequences.

doc/ssl/SSL_CTX_set1_curves.pod around line 90: =back without =over
doc/ssl/SSL_CTX_set1_verify_cert_store.pod around line 73: =back without =over
doc/ssl/SSL_CTX_add1_chain_cert.pod around line 82: =back without =over
doc/crypto/evp.pod around line 40: '=item' outside of any '=over'
crypto/des/des.pod around line 184: You forgot a '=back' before '=head1'

PR#3147

Preliminary documentation for chain and verify stores and certificate chain
setting functions.