dh-exec.lib.c: Do not filter out scripts when DH_EXEC_CMD_ALWAYS is set

When DH_EXEC_CMD_ALWAYS is set at build time, return from
dh_exec_script_allowed() early. This is a much cleaner solution to the
always-strip problem than the previous one (which is now reverted).

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
debian/changelog: Finalised the date.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>