1. If not building with PAM, kcheckpass relies on HAVE_SHADOW to decide
whether to support shadow passwords. However, this was only set if also
building tdm.
Consolidate all PAM/shadow configure checks at the top level so these are
always set correctly.
2. Consolidate /etc/passwd and shadow password handling
The shadow password handler already completely handles /etc/passwd
passwords as well, so having a separate handler for just /etc/passwd is
pure code duplication.
Signed-off-by: Bobby Bingham <koorogi@koorogi.info>