configure.ac: be deterministic about passwd location

configure.ac: be deterministic about passwd location

Statically set PASSWD_PROGRAM depending on exec_prefix, and not by where
the passwd program was at configure time.

Depending on the specific build situation before, this may or may not
change the embedded passwd program path. Also configure.ac sets
exec_prefix=/ for prefix=/usr, so this might be a bit confusing, but
at least deterministic.

Closes: #1224
Signed-off-by: Chris Hofstaedtler <zeha@debian.org>