lib/obscure.c: Introduce obscure_get_range

lib/obscure.c: Introduce obscure_get_range

Unify the retrieval of PASS_MIN_LEN and PASS_MAX_LEN for output
in passwd and actual checks.

Fixes wrong output for minimum password lengths if no such
restriction is configured: 5 is printed, 0 is in effect.

How to reproduce:

1. Use passwd compiled without PAM support
2. Do not specify PASS_MIN_LEN in login.defs
3. Run passwd as a user and enter your old password, then
   - you will see that 5 characters are expected
   - you can just press enter twice

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>