* configure.in: check for <linux/socket.h>, which, if found,

* configure.in: check for <linux/socket.h>, which, if found,
appears to be preferable to <sys/socket.h>.  Use it to find
<linux/nfs_mount.h>.  On some Linux systems
(Gentoo), both exist but only one must be included, or else you
get redefined structure errors.

* libamu/wire.c (is_network_member): check if masknum is
INADDR_NONE, not "< 0" because it's an unsigned quantity.

* libamu/util.c (xstrlcpy): remove useless test for size_t < 0,
because it's an unsigned quantity.