pam_unix: make configuration consistent in --enable-static-modules mode

pam_unix: make configuration consistent in --enable-static-modules mode

In --enable-static-modules mode, it was not possible to use "pam_unix"
in PAM config files.  Instead, different names had to be used for each
management group: pam_unix_auth, pam_unix_acct, pam_unix_passwd and
pam_unix_session.  This change makes pam_unix configuration consistent
with other PAM modules.

* README: Remove the paragraph describing pam_unix distinctions in
--enable-static-modules mode.
* libpam/pam_static_modules.h (_pam_unix_acct_modstruct,
_pam_unix_auth_modstruct, _pam_unix_passwd_modstruct,
_pam_unix_session_modstruct): Remove.
(_pam_unix_modstruct): New pam_module declaration.
* modules/pam_unix/pam_unix_static.h: New file.
* modules/pam_unix/pam_unix_static.c: Likewise.
* modules/pam_unix/Makefile.am (noinst_HEADERS): Add pam_unix_static.h
(pam_unix_la_SOURCES) [STATIC_MODULES]: Add pam_unix_static.c
* modules/pam_unix/pam_unix_acct.c [PAM_STATIC]: Include
pam_unix_static.h
[PAM_STATIC] (_pam_unix_acct_modstruct): Remove.
* modules/pam_unix/pam_unix_auth.c [PAM_STATIC]: Include
pam_unix_static.h
[PAM_STATIC] (_pam_unix_auth_modstruct): Remove.
* modules/pam_unix/pam_unix_passwd.c [PAM_STATIC]: Include
pam_unix_static.h
[PAM_STATIC] (_pam_unix_passwd_modstruct): Remove.
* modules/pam_unix/pam_unix_sess.c [PAM_STATIC]: Include
pam_unix_static.h
[PAM_STATIC] (_pam_unix_session_modstruct): Remove.

Suggested-by: Matveychikov Ilya <i.matveychikov@securitycode.ru>