* tools/nss/libvirt_nss.[ch]: add BSD-comptabile wrappers and
register via the nss_module_register() interface
* m4/virt-nss.m4: add checks if we're building NSS for FreeBSD
* tools/Makefile.am: handle target library name differences, as
Linux needs libnss_libvirt.so.2 and FreeBSD needs
nss_libvirt.so.1. Also, different syms files have to be used
as Linux needs to export all the methods while FreeBSD
only needs to have nss_module_register()
* tests/nsstest.c, tests/nssmock.c: s/__linux__/NSS/
* tests/nssmock.c: pass int instead of mode_t to va_arg() to please
gcc 4.8
* libvirt_nss_bsd.syms: FreeBSD syms file
