maint.mk: detect missing-NL-at-EOF, too

maint.mk: detect missing-NL-at-EOF, too

* top/maint.mk (sc_prohibit_empty_lines_at_EOF): Adjust so that
it also detects when a file lacks a newline at EOF.
(require_exactly_one_NL_at_EOF_): Renamed from
detect_empty_lines_at_EOF_.  I opted not to rename the rule,
since people may well have .x-sc_... file names tied to the
existing name.  Suggested by Eric Blake.