* configure.in: Added option --enable-account-tools-setuid to

	* configure.in: Added option --enable-account-tools-setuid to
	enable/disable the usage of PAM to authenticate the callers of
	account management tools: chage, chgpasswd, chpasswd, groupadd,
	groupdel, groupmod, useradd, userdel, usermod.
	* src/Makefile.am: Do not link the above tools with libpam if
	account-tools-setuid is disabled.
	* src/userdel.c, src/newusers.c, src/chpasswd.c, src/usermod.c,
	src/groupdel.c, src/chgpasswd.c, src/useradd.c, src/groupmod.c,
	src/groupadd.c, src/chage.c: Implement ACCT_TOOLS_SETUID
	(--enable-account-tools-setuid).
	* etc/pam.d/Makefile.am: Install the pam service file for the
	above tools only when needed.
	* src/useradd.c, src/userdel.c, src/usermod.c: It is no more
	needed to initialize retval to PAM_SUCCESS.