Fix npth port file handle use.
* assuan.h.in (ASSUAN_SYSTEM_NPTH_IMPL): Use npth_unprotect and npth_protect with actual assuan functions instead of npth wrappers. * libassuan.def: Add __assuan_read, __assuan_write, __assuan_sendmsg, __assuan_recvmsg, __assuan_waitpid exports. * system-w32.c (__assuan_read, __assuan_write, __assuan_sendmsg, __assuan_recvmsg, __assuan_waitpid): Make non-static.