evdns: accept domains up to 254 long (previosly only 63 long was accepted)

evdns: accept domains up to 254 long (previosly only 63 long was accepted)

Previously evdns was using HOST_NAME_MAX, and define it to 255 *only* if
it not set, however it does set on linux:

    $ egrep -r define.*HOST_NAME_MAX /usr/include/bits
    /usr/include/bits/local_lim.h:#define HOST_NAME_MAX             64
    /usr/include/bits/posix1_lim.h:#define _POSIX_HOST_NAME_MAX     255
    /usr/include/bits/confname.h:#define _SC_HOST_NAME_MAX          _SC_HOST_NAME_MAX

But 64 should be the limit of the host component, not for the whole
hostname, as also noted by @ploxiln

So use our own EVDNS_NAME_MAX const, which is set to 255.

Fixes: #1280