bootstrap: split into reusable parts.

bootstrap: split into reusable parts.

* bootstrap: Bourne shell has no scoping, so be extremely
careful with namespaces, functions in `^func_' and `^require_';
private variables in `^_G_', and public variables all well
documented by comments.
* libltdl/config/extract-trace: New file, containing the minimum
of code previously in bootstrap plus a little glue to make the
GNU M4 based autotools tracing function (as opposed to the
kludgy sed extraction currently used everywhere else) a
standalone script that can be executed or sourced.
* libltdl/config/options-parser: New file, containing the
pluggable options parser code shared between the other two.
* bootstrap.conf: Adjust.
* Makefile.am (EXTRA_DIST): Be sure to distribute the new
extract-trace and options-parser scripts.
(install-data-local): And install them where libtoolize can find
them.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>