FreeBSD winbind support is broken: as compiled, the nss_winbind.so module is 
missing the actual winbind symbols which perform the getpwnam/getgrent/etc 
functions. winbind_nss_freebsd.c is not intended as a replacement but as a 
wrapper around the Linux code. It still needs the functions in 
winbind_nss_linux.c.

To observe the bug:

1) Compile and install the nss_winbind.so module, and edit nsswitch.conf to 
use winbind.

2) Use pw showuser [DOMAIN USER], smbd, etc. Winbind clearly does not work, 
and you'll get messages similar to the following in the debug syslog facility:

NSSWITCH(nss_method_lookup): winbind, group, setgrent, not found
NSSWITCH(nss_method_lookup): winbind, group, getgrent_r, not found
NSSWITCH(nss_method_lookup): winbind, group, endgrent, not found

...ad infinitum.

Attached is a tiny patch to Samba's configure script to fix the problem. After 
you recompile and reinstall libnss_winbind.so (as nss_winbind.so in /usr/lib) 
it works and the messages cease.