util: Fix build on FreeBSD by avoiding NSS_BUFLEN_PASSWD

util: Fix build on FreeBSD by avoiding NSS_BUFLEN_PASSWD

NSS_BUFLEN_PASSWD is not defined on FreeBSD.  Use
sysconf(_SC_GETPW_R_SIZE_MAX) instead, as per POSIX.

Use a dynamically allocated buffer instead of trying to cram all of
the logic into the declarations.  This will come in useful later
anyway.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Bjoern Jacke <bjacke@samba.org>
(cherry picked from commit 847208cd8ac68c4c7d1dae63767820db1c69292b)