shorten Tls_lwt.rng_init

* Lwt_cstruct.complete
* read 32 bytes instead of 16
* ~rng_file -> ~device
tls_lwt: periodic refeeding of entropy
++ cstruct.lwt
default to reseeding
simple prototype of sleep-preserving rng reseeder
Merge pull request #187 from mirleft/lwt-continuous-rng

Continuously reseed rng with lwt/unix
describe approach of SNI
adjust interface of configuration
adjust implementation and checks of configuration
first select certificate (depending on SNI from client), then ciphersuite
fix compilation of lwt and examples
provide a polymorphic variant for own certificates
cleanup after type change in config.own_certificates
minor updates
example with SNI
provide interface to extract epoch from tls_lwt.Unix.t
regenerate
update
fix mirage examples
note about SNI