Fix deletion on invalid symlinks.

It seems that Sys.file_exists returns false if a symbolic link exists but is broken. To avoid this, we also use Unix.lstat to be completely sure.

Fix #520
Fix user-configuration in case of syntax error (for instance when ~/.profile is run by dash)
Update CHANGES
Quiet OPAM init scripts (thx to @cago)
Propose to update ~/.profile if it does not exist
Fix update of ~/.profile to source the right ~/.opam/opam-init/init.{sh,csh,zsh} or not.
Fix the message after opam-init to source right init script (sh/csh/zsh)
Update the documentation for --global
Expand ~ in filename strings

This fixes `opam config setup --dot-profile=~/.bashrc`

Remark: this was already working fine if you do `opam config setup --dot-profile ~/.bashrc` (as this was shell-expanded)
Uddate CHANGES
make the git stamp more robust against not having any local annotated tags.  Now any failure results in a valid file for the version
Improve error message in case of checksums on 'opam init'
Update doc API
Update man pages
Fix silly regression introduced in de774165518c9c2abdeaf63a68081738fce1e774
Add a 'release' target to facilitate releases
Fix release target
Merge pull request #521 from avsm/master

make the build more robust to git failure
Merge branch 'master' of github.com:OCamlPro/opam
Update the release/upload targets