kex: server fix to include 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.

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'.