doc: Update documentation of ssh_set_blocking().

This should work correctly in libssh 0.6.0. If not then you hit a bug.
doc: Improve the PKI documentation a bit.
cmake: Allow to build without examples.

BUG: https://red.libssh.org/issues/114
channel: Document SSH_AGAIN in ssh_channel_read().

BUG: https://red.libssh.org/issues/115
channels: Correctly decrement timeout value in ssh_channel_accept().

BUG: https://red.libssh.org/issues/116
client: Add example code for ssh_get_openssh_version().

BUG: https://red.libssh.org/issues/120
session: Remove obsolete status variables.

BUG: https://red.libssh.org/issues/121
session: Document return value of ssh_get_serverbanner().

BUG: https://red.libssh.org/issues/122
callbacks: Improve the documentation of ssh_threads_get_noop().

BUG: https://red.libssh.org/issues/123
callbacks: Improve the documentation of ssh_threads_set_callbacks().

BUG: https://red.libssh.org/issues/123
channel: Use the correct timeout option in channel_open().

BUG: https://red.libssh.org/issues/124
channels: Correctly handle timeouts in channel functions.
kex: implement curve25519-sha256@libssh.org
doc: Documentation of curve25519-sha256@libssh.org
callbacks: add support for auth_none_function
session: Try the ecdsa default key first.
channel: Fix packets termination timeout in global_request().

BUG: https://red.libssh.org/issues/126
doc: Make sure we have the defines to build all docs.
client: Added a missing NULL pointer check.

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
scp: Fixed result of ssh_scp_string_mode() to get SCP working.

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>