maint: describe new versioning and branching scheme, and adjust to it

maint: describe new versioning and branching scheme, and adjust to it

See discussion about automake bug#13578 for more details and background.

Basically, for the versioning scheme:

  - micro versions only for bug and regression fixing;
  - minor versions for new backward-compatible features, and new
    non-fatal deprecations;
  - major versions for backward-incompatibilities, complex new
    features, and major refactoring.

And for the git branching scheme:

  + branch 'next' is for the upcoming major version;
  + branch 'master' is now for the upcoming minor version;
  + branch 'maint' is for the upcoming micro (bug-fixing) version;
  + the merging hierarchy is: 'maint' -> 'master' -> 'next'.

* HACKING (Automake versioning and compatibility scheme): New.
(Working with git): Adjust.
* NEWS: Update and fix.
* aclocal.in: Adjust some "FIXME" messages.
* automake.in: Likewise.
* m4/mkdirp.m4: Likewise.
* t/aclocal-acdir.sh: Likewise.
* t/aclocal-macrodir.tap: Likewise.
* t/aclocal-macrodirs.tap: Likewise.
* lib/Automake/Options.pm: Likewise.
* m4/internal/ac-config-macro-dirs.m4: Likewise.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>