maint: copy FDL from gnulib instead of using it as module

Since the previous gnulib update, bootstrap outputs this warning:

  Notice from module fdl:
    Don't use this module! Instead, copy the referenced license file \
    into your version control repository.

See gnulib commit:
  https://git.sv.gnu.org/cgit/gnulib.git/commit/?id=88fc5afbccc9

* bootstrap.conf (gnulib_modules): Remove 'fdl'.
* doc/fdl.texi: Add file as a copy of 'gnulib/doc/fdl.texi'.
* doc/.gitignore (/fdl.texi): Remove entry.
* cfg.mk (FILTER_LONG_LINES): Add pattern for the 'fdl.texi' file.

bootstrap.conf

@@ -82,7 +82,6 @@ gnulib_modules="
   fcntl-safer
   fd-reopen
   fdatasync
-  fdl
   fdopen
   fdutimensat
   file-has-acl