Make the PAM password strength checking module WARNS=2 safe.

lib/libpam/modules/pam_passwdqc/Makefile:
	Bump WARNS to 2.

contrib/pam_modules/pam_passwdqc/pam_passwdqc.c:
	Bump  _XOPEN_SOURCE and _XOPEN_VERSION from 500 to 600
	so that vsnprint() is declared.

	Use the two new union types (pam_conv_item_t and
	pam_text_item_t) to resolve strict aliasing violations
	caused by casts to comply with the pam_get_item() API taking
	a "const void **" for all item types.  Warnings are
	generated for casts that create "type puns" (pointers of
	conflicting sized types that are set to access the same
	memory location) since these pointers may be used in ways
	that violate C's strict aliasing rules.  Casts to a new
	type must be performed through a union in order to be
	compliant, and access must be performed through only one
	of the union's data types during the lifetime of the union
	instance.  Handle strict-aliasing warnings through pointer
	assignments, which drastically simplifies this change.

	Correct a CLANG "printf-like function with more arguments
	than format" error.

Submitted by:	gibbs
Sponsored by:	Spectra Logic
Add note/reminder about dialog(1) regression in HEAD/10.0-C so that we don't
forget about it in the multi-month run of things to fix prior to 10.0-R.