Alarm handler should exit with status NO

The alarm signal handler should exit the program with the NO status
code. This is already done, but it uses a constant instead of the enum
that defines CHECK_NO. Switch to CHECK_NO so that it uses the value that
CHECK_NO uses, even if that changes.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Allow PAM service to be specified

We use PACKAGE_NAME as the name of the PAM service. This name won't
necessarily work everywhere and it is likely that users want to specify
it. Let users specify it using a variable in configure.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>