The loadable sesman authentication modules use different types for the
authentication handle returned from auth_userpass(). The PAM module
uses a pointer, and the other modules use (effectively) a boolean. Within
sesman itself, a long or tbus (intptr_t) is used.
This PR replaces all of these types with a pointer to an incomplete type.
Consequently:-
- A single better-labelled type is used it all places within sesman so
it's more obvious what's being handled.
- There is no need to cast the authentication handle within the PAM
module to a long and back again.
- The compiler can check function signatures between auth.h and the
various verify modules.
Include string.h for strcmp(). Don't define _XOPEN_SOURCE, it conflicts
with libc headers and hides putpwent(). Make input strings constant. Fix
functions to match their declarations.
