lib: remove CONSTCOND comment

Since 2021-01-31, lint doesn't need it anymore for the common pattern of
'do ... while (0)'.

move the host file getbyname and getbyaddr functions to sethostent.c so
that they are in a separate file from the dns ones in order for libhack
to use them.

Add not advertised reentrant functions: {get,set,end}hostent_r,
gethostbyname{,2}_r, gethostbyaddr_r. Make getnameinfo(3) use
gethostbyaddr_r(3) so it is re-entrant (ahem __ypdomain). These
are not b

Add not advertised reentrant functions: {get,set,end}hostent_r,
gethostbyname{,2}_r, gethostbyaddr_r. Make getnameinfo(3) use
gethostbyaddr_r(3) so it is re-entrant (ahem __ypdomain). These
are not being advertised because there is a bunch of different
implementation of them that have a variety of type signatures.

If people want to follow someone's implementation, it is now easy.

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