build: fix generation of manpages for programs not built by default

build: fix generation of manpages for programs not built by default

* src/local.mk (dist-hook): Don't use this to ensure all the
programs, even the ones disabled by default or by the user, are
built (doing so is required to ensure the distributed manpages
are properly built).  This would build those programs too late
anyway, causing errors like:

    $ make dist
    make  dist-xz am__post_remove_distdir='@:'
    make[1]: Entering directory `~/src/coreutils'
      GEN      man/arch.1
    help2man: can't get '--help' info from man/arch.td/arch
    make[1]: *** [man/arch.1] Error 127
    make[1]: Leaving directory `~/src/coreutils'
    make: *** [dist] Error 2

Instead, ...
* man/local.mk (extra_man_x, extra_man_1): define these ...
($(extra_man_1)): ... and make this depend on $(all_programs).
(EXTRA_DIST): Adjust.
Adjust some comments as well.