[PATCH] replace RFC2181 magic numbers with POSIX definitions

The maximum length of host names is provided by POSIX-compliant
compilers. MAXHOSTNAMELEN is a standard definition relative to those
values so we do not need the magic numbers. The actual RC2181 texts
describe the same limits defined by POSIX IEEE Std 1003.1.

There are some slightly-broken systems which use an older POSIX standard
with much smaller values. But I feel we should obey those system limits
and not pass their kernels larger chunks of data.

This makes use of the standard values instead of our magic numbers.

Votes?

Amos