Server-side support for encrypt-then-MAC extension (RFC 7366)

The new TLS extension type 0x16 is used to negotiate encrypt-then-MAC
between client and server. For AEAD cipher suites, this extension can
be skipped by the server. As long as no other cipher suites are
supported by this library, it can be ignored.
Fixed typo in console output.
Fixed even more typos and a misleading diagnostic message.
Bugfix: error result in get_psk_info() callback

get_psk_info() is called during handshake to select a PSK that
is associated with the given PSK identity. A positive result indicates
the actual number of bytes written to the result parameter.
A negative value indicates an error.

As the codes starting with DTLS_ALERT_ are positive values, they
must be transformed with dtls_alert_fatal_create() before return.

This bugfix addresses ticket #5 "PSK identity is not checked".