MAJOR: dns: runtime resolution can change server admin state

MAJOR: dns: runtime resolution can change server admin state

WARNING: this is a MAJOR (and disruptive) change with previous HAProxy's
behavior: before, HAProxy never ever used to change a server administrative
status when the DNS resolution failed at run time.

This patch gives HAProxy the ability to change the administrative status
of a server to MAINT (RMAINT actually) when an error is encountered for
a period longer than its own allowed by the corresponding 'hold'
parameter.

IE if the configuration sets "hold nx 10s" and a server's hostname
points to a NX for more than 10s, then the server will be set to RMAINT,
hence in MAINTENANCE mode.