libreplace: Add tests for connect and gethostbyname.

libreplace: Add tests for connect and gethostbyname.

Provide dummy replacements when a function isnt found.
The functions are also searched for in certain libraries,
and variables SOCKET_LIBS and NSL_LIBS are set accordingly.

One purpose of this is to fix the getifaddrs tests on
systems where e.g. the socket calls require special libs
for linking.

Michael
(This used to be commit c19f7a0e1004213f95e0bf8db5cd1f6697c7a47b)