gnulib-tool: new options --add-import, --remove-import

[Bruno Haible]

Hi,

gnulib-tool is being used in two contexts:
  A) Directly by a user, who wants to type the minimum of arguments.
  B) By an autogen.sh or bootstrap script, where it is important to
     keep a clear separation between files that are "input" (stored in VCS)
     and files that are "output" (not in VCS).

Currently, "gnulib-tool --import" is trying to satisfy both use cases,
but does not satisfy either of them fully:
  - User A has no way to undo an --import of a module without modifying
    gnulib-cache.m4 by hand.
  - User B has to remove gnulib-cache.m4 before running gnulib-tool.

I would propose to
  - Rename the current --import option to --add-import,
  - Introduce a new --import option, that is like the previous --import
    option except that it ignores the contents of gnulib-cache.m4,
  - Add a --remove-import option that can be used to remove a module
    from the list of modules (before dependency computation).

'bootstrap' would not have to change, because it already invokes
'gnulib-tool --import'.

Opinions? Objections?

Bruno