* Remove unused bindings from asn1.py

This also includes a couple removals from x509v3.py which also reference
ASN1_ITEM_EXP.

* re-add int ASN1_STRING_set_default_mask_asc(char *);

* also re-add static const int MBSTRING_UTF8

Currently we only expose SSL_get_peer_certificate, which allows you
to retrieve the remote party's certificate. This adds the symetrical
binding to retrieve the local party's certificate.

The motivation for this additional binding is to make it possible to
query the local certificate regardless of the method which was used to
load the certificate into the SSL connection (from a file, from an
in-memory object). An example where this is useful is when negotiating
a DTLS-SRTP connection, the fingerprint of the local certificate needs
to be communicated to the remote party out-of-band via SDP.

This binding can be exposed in pyopenssl as Connection.get_certificate().

* + more DTLS bindings

* + BIO_CTRL_DGRAM*

* + read ahead functions

* rm BIO_CTRL_DGRAM_SET_PEEK_MODE

* rm BIO_CTRL_DGRAM_SET_DONT_FRAG

* + link mtu conditional logic

* rm some BIO_CTRL_DGRAM* bindings

* + PSK function bindings

* + PSK conditional

* trigger CI

* trigger CI

* + bindings for SSL_OP_NO_DTLS*

* + conditional for not HAS_GENERIC_DTLS_METHOD

* flag SSL_OP_NO_DTLS* for unsupported deletion

This avoids reaching into the ASN1_ITEM mess if not necessary.