Add support to change HOST_NAME constant

[sezeryalcin]

There is absolutely no point of keeping a static hostname prefix as this library is free and open source. There are already made patches for allowing customization of DHCP hostname string so please allow this functionality.

Dhcp.h

#define HOST_NAME "WIZNet"

I am seeing following in my DHCP table which could make much sense if I had a chance to customize it.

WIZnetEFFEEC | DE:AD:BE:EF:FE:EC | 192.168.1.107 | 47:49:21
WIZnetEFFEEB | DE:AD:BE:EF:FE:EB | 192.168.1.108 | 47:50:28
WIZnetEFFEEA | DE:AD:BE:EF:FE:EA | 192.168.1.110 | 47:58:33
WIZnetEFFEE0

[Russell108]

Hello Arduino team I have many Arduino on my network If I can set the hostname as in the wifinina library, it would be really easy to find them on my DHCP server
It's not a trivial thing but really useful for administration

[Russell108]

OK For me, this is no longer such a big issue I realise that the hostname is unique
I understand that the default will be WIZnet, followed by the last three octets of the mac address.

[thirstyice]

See my PR #248

[ThSchwanck]

I needed an option to set non constant host names, see my PR #257.