POD: Fix list termination

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