Provide a replacement for socklen_t.
* m4/socklen.m4, m4/sys_socket_h.m4: New. * configure.ac (INSERT_SOCKLEN_T): New. * src/npth.h.in (INSERT_SOCKLEN_T): New substitution. (npth_connect, npth_accept): Use _npth_socklen_t.
For example SGI IRIX 6.5 MIPS R10000 does not provide socklen_t. We
provide the usual fallback to int as figured out by a gnulib test.
Note that _npth_socklen_t is internal to nPth and shall not be used by
applications.