kex: server fix for first_kex_packet_follows

Ensure to honor the 'first_kex_packet_follow' field when processing
KEXINIT messages in the 'ssh_packet_kexinit' callback.  Until now
libssh would assume that this field is always unset (zero).  But
some clients may set this (dropbear at or beyond version 2013.57),
and it needs to be included when computing the session ID.

Also include logic for handling wrongly-guessed key exchange algorithms.
Save whether a client's guess is wrong in a new field in the session
struct: when set, the next KEX_DHINIT message to be processed will be
ignored per RFC 4253, 7.1.

While here, update both 'ssh_packet_kexinit' and 'make_sessionid' to
use softabs with a 4 space indent level throughout, and also convert
various error-checking to store intermediate values into an explicit
'rc'.

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
cmake: Fix doxygen.
session: Fix a memory leak with custom banner.

BUG: https://red.libssh.org/issues/152
channels: Fix exit-signal request.

BUG: https://red.libssh.org/issues/153
doc: Improve docs for ssh_channel_get_exit_status().

BUG: https://red.libssh.org/issues/154
examples: Fix a bad shift if ssh_get_fd() returns -1.

Found by Coverity.

CID: #1199454