Make some operators in IPAddress const

These functions do not modify the IPAddress object, but were not marked
as const. This meant that you could not do:

void set_ip(const IPAddress& ip) {
	uint32_t copy = ip;
}

Since calling operator uint32_t() on ip would discard the constness of
the reference.
Merge remote-tracking branch 'matthijs/ide-1.5.x-ipaddress-const' into ide-1.5.x
Merge branch 'ide-1.5.x' into ide-1.5.x-avr-toolchain