libreplace: replace inet_ntoa() when it is missing

libreplace: replace inet_ntoa() when it is missing

...not only replace it when it is broken.

This moves the defintion of rep_inet_ntoa from replace.c
to inet_ntoa.c and adds configure checks for existence
of inet_ntoa(). Checks are moved to an include file of its own.

NOTE: The original rep_inet_ntoa in replace.c was wrapped
into a "#ifndef WITH_PTHREADS" but the prototype in replace.h
and the define in system/network.h were not. I removed that
ifndef since the inet_ntoa() function is usually not thread safe
anyways, since it returns a pointer to a static buffer.

So whoever calls inet_ntoa() should be aware that it is not
thread safe anyways.

Michael
(cherry picked from commit 974c0c45ad42644348e0b55454715b12158f1028)