maint.mk: fix strict vs. lazy variable issues with RELEASE

maint.mk: fix strict vs. lazy variable issues with RELEASE

* top/maint.mk (_equal): New function.
(member_check): Strip the result to avoid spurious spaces.
(url_dir_list): Do not use ifeq, which is strict, as it will
require RELEASE_TYPE to be defined.
(announcement_Cc_, announcement_mail_headers_): Likewise: instead
of relying on ifeq, use $(release_type) to dispatch (lazily) onto...
(announcement_Cc_alpha,announcement_mail_headers_alpha)
(announcement_Cc_beta,announcement_mail_headers_beta)
(announcement_Cc_stable,announcement_mail_headers_stable): these.
(release): Do not depend on $(release-type), as it forces its
evaluation.  Bounce to it.