maint: use more sc_-prefixed names

maint: use more sc_-prefixed names

* maint.mk (sc_po_check): Renamed from po-check.
(sc_author_mark_check): Renamed from author_mark_check.
(sc_copyright_check): Renamed from copyright-check.
(sc_makefile_path_separator_check): Renamed from
makefile_path_separator_check.
(local-checks-available): Remove the old names.
* Makefile.am (EXTRA_DIST): Reflect the renaming: .x-sc_po_check
* .x-po-check: Rename to...
* .x-sc_po_check: ...new file.