Zabbix

.......PS. [ZBX-4857] fix poller hanging on ssh check when using public key authentication
	   [svn merge ^/branches/1.8 -c27192]

libssh2 hangs if libssh2_userauth_publickey_fromfile() is called with
public key parameter as directory. There is a bug report made by
zalex:

http://www.libssh2.org/mail/libssh2-devel-archive-2012-04/0021.shtml

Until it's fixed we do a regular file check before doing an libssh2
call. Symbolic links as key files are supported.

This commit also adds more error information when libssh2 call fails.