Ensure build-time finishing after Makefile changes

Due to a typo, the dependency rule that triggers build-time finishing
after an (implicit) change of Makefile wasn't applied anymore.
Install libraries in a decent place, i.e. $(libdir)

Unfortunately, GNU Automake is quite strict about its primaries. Thus,
actually installing demand-loadable POSIX shell libraries in $(libdir)
requires some trickery because
  - trying lib_LIBRARIES=... makes automake(1) complain about an unknown
    suffix `.sh' and pull in the whole compile/link machinery, whereas
  - trying lib_DATA makes automake(1) complain about in invalid primary
    combination.

Therefore, we emulate lib_DATA by introducing user-defined installation
directory $(shlibdir) which defaults to $(libdir) and using the primary
shlib_DATA.

To deal with possible build-time overrides of $(shlibdir), we use the
placeholder @shlibdir@ throughout the code (although we actually *mean*
@libdir@, as you can see in `libdl-sh echo libdir').
Rename LIBSH_FINISH_FILES() to AF_FINISH_FILES()

This patch is the first in a series to clarify the terminology and prepare
separation of the build-time finishing framework into its own project.

In order to do so, we
  - rename M4 macro LIBSH_FINISH_FILES() to AF_FINISH_FILES() and
  - start talking about *finishing* instead of finalization.
Rename AF_FINISH_FILES()'s internal variables

This patch is the second in a series to clarify the terminology and prepare
separation of the build-time finishing framework into its own project.

We consider the build-time finishing framework being an extension of the
GNU Autotools. Consequently, we rename variables
  - libsh_cleanfiles         to af__CLEAN_FILES and
  - libsh_config_status_deps to af__config_status_deps,
mimicing automake(1)'s naming conventions. Both of them get ultimately
AC_SUBST()ed into Makefile.

Following the same conventions, we also rename variables
  - libsh_gensubst   to af__gensubst and
  - libsh_finalizees to af__unfinished.
The latter is also done for clarifying terminology.
Rename output variable FINALIZE to FINISH

This patch is the third in a series to clarify the terminology and prepare
separation of the build-time finishing framework into its own project.

It renames output variable FINALIZE to FINISH (because that's what it does
now). For claryfing terminology in auto/gensubst, variable finalizees is
also renamed to unfinished.
Move auto/finalize.{am,m4} to auto/autofinish.{am,m4}

This patch is the fourth in a series to clarify the terminology and prepare
separation of the build-time finishing framework into its own project.

For purposes of preparing project separation, it moves
  - auto/finalize.am to auto/autofinish.am and
  - auto/finalize.m4 to auto/autofinish.m4.
It also updates any references to these files, thus completing the series.
Add note about how to invoke AF_FINISH_FILES()

In a note, mention that all pathnames passed to AF_FINISH_FILES()
must be relative to $(top_srcdir).