* src/chgpasswd.c: Add annotations to indicate that usage() does

	* src/chgpasswd.c: Add annotations to indicate that usage() does
	not return.
	* src/chgpasswd.c: Split usage in smaller parts. Those parts are
	already translated for chpasswd. Usage is now closer to
	chpasswd's.
	* src/chgpasswd.c: Remove dead code. No need to set crypt_method
	to NULL when it is already NULL. sflg is only set if crypt_method
	is not NULL.