Use "git archive" for the "make releasetar" process

Use "git archive" for the "make releasetar" process

Use the release tag if it exists or use HEAD.

Remove the "autoreconf -f" command, because it changes the configure file
locally in an uncontrolled way (runstatedir, #define LARGE_OFF_T, etc.)
depending on the autoconf version. This command is run in the release
process before a commit and we can choose the parts that will be added
in the commit.

Note:
The following target (rcX) must be updated ou removed in a next step
because it was using "autoreconf -f" (via "make releasetar") and it
changes the VERSION and configure files locally.

This change
1) Ensures that we only release files from tag/HEAD, not locally
   modified ones.
2) Avoids disclosing personal data such as the username/group of the
   local user.
3) Puts by default a umask of 0002, which turns off the world write bit
   on files in the archive.
4) Avoids problems on some OSes (no more tar, Git builtin tar.gz
   handling).