* feature: restrict symbols which can exit the interpreter process

Some symbols such as os.Exit or log.Fatal, which make the current process
to exit, are now restricted. They are replaced by a version which panics
instead of exiting, as panics are recovered by Eval.

The restricted os.FindProcess version is identical to the original
except it errors when trying to return the self process, in order to
forbid killing or signaling the interpreter process from script.

The os/exec symbols are available only through unrestricted package.

The original symbols are stored in an unrestricted package, which
requires an explicit Use, as for unsafe and syscall packages.

The Use() interpreter method has been slightly modified to allow inplace
updating of package symbols, allowing to replace some symbols but not
the entire imported package.

A command line option -unrestricted has been added to yaegi CLI to use
the unrestricted symbols.

Fixes #486.

* fix: lint