maint: fix and simplify maintainer checks

maint: fix and simplify maintainer checks

Some of them can be simplified after the previous changes, some
of them have been downright broken by them, and need fixing.

* src/local.mk: Adjust some comments.
(EXTRA_DIST): Avoid SPACE-TAB sequences.
(src/dircolors.h, src/fs.h src/fs-is-local.h): Avoid 8-SPACES
indentation.
(_sc_check-AUTHORS): Move ...
* cfg.mk (sc_check-AUTHORS): ... here (superseding the old rule
with the same name, that was just a recursive invocation to it).
Adjust the paths of the invoked coreutils programs, to account
for the fact that this rule now runs in the top-level build dir,
not in the 'src/' subdir.  Other minor cosmetic adjustments.
(ALL_RECURSIVE_TARGETS): Remove 'sc_option_desc_uppercase' and
'sc_man_file_correlation', since they no longer entail any
recursive make invocation.
(sc_option_desc_uppercase): Remove dependency from $(all_programs):
it isn't actually needed.
(check-programs-vs-x): Likewise.  Also, fix heading comments to
truly reflect what this check does.
(all-progs-but-lbracket): Strip the 'src/' prefix from each entry
in the list of programs; this avoids a spurious failure in the
'check-programs-vs-x' recipe.
(.PHONY): No need to list targets 'sc_man_file_correlation' and
'sc_option_desc_uppercase': they are automatically declared phony
by 'maint.mk', being recognized as syntax checks.