Quick fix: define E_PAM_ERR in lib/pam_pass.c

Quick fix: define E_PAM_ERR in lib/pam_pass.c

The exit code situation is a hot mess. Do a
  git grep "define.*E_SUCCESS"
Each src/*.c is defining its own set of error codes, and
they are frequently conflicting, e.g. more than one use
10.

We should probably have a common set defined in lib/exitcodes.h.
I'm thinking for a first cut, we just move all the definitions
from src/*.c to lib/exitcodes.h, and let the conflicts stand.
If we later want to change some defines to make them unambiguous
across the project, we can do that separately.

Signed-off-by: Serge Hallyn <serge@hallyn.com>