TcpServer: optimize DNS lookups for PTR
TcpServer: optimize DNS lookups for PTR

a. don't use search path (/etc/resolv.conf)
b. explicitely specify PTR in query request

sieutruc/qpsmtpd,priyadi/qpsmtpd,priyadi/qpsmtpd,smtpd/qpsmtpd,priyadi/qpsmtpd,sieutruc/qpsmtpd,msimerson/qpsmtpd,salvis/qpsmtpd,sieutruc/qpsmtpd,salvis/qpsmtpd,msimerson/qpsmtpd,smtpd/qpsmtpd