Disable Kerberos and NTLM auth for mail fetch

On some configurations of PHP and remote mail servers, Kerberos and NTLM
challenge response authentication could be possibly attempted. This is
essentially futile and potentially fatal for osTicket mail fetching, as the
system is currently designed for username and password authentication only.

This patch disables challenge and response authentication for PHP versions
5.3.2 and newer, which support the fix.

This patch also consistently encodes mailbox names according to the rfc 2060
for IMAP.