libssh2_knownhost_checkp took 0 as a magic port number that indicated
a 'generic' check should be performed. However, 0 is a valid port
number in its own right so this commit changes the magic value to any
negative int.
OpenSSH has ways to add hosts to the knownhosts file that include
a specific port number which makes the key associated with only
that specific host+port pair. libssh2 previously did not support
this, and I was forced to add a new function to the API to
properly expose this ability to applications:
libssh2_knownhost_checkp()
To *add* such hosts to the knownhosts file, you make sure to pass
on the host name in that manner to the libssh2_knownhost_addc()
function.
