Instead of hardcoding the location of perl (assuming it is installed in
/usr), use /usr/bin/env to run it, and thus picking it from $PATH.
This makes it possible to run these scripts also on installations with
perl in a different prefix than /usr.
Also, given that we want enable warnings on scripts, turn the -w
previously in shebang to explicit "use warnings;" in scripts which
didn't have it before.
In libguestfs 1.20, you will be able to use 'add_drive'
instead of 'add_drive_opts' (except in the C bindings).
However until libguestfs 1.20 is the minimum stable version
people will still be using old versions where you have to use
'add_drive_opts'. This makes the examples confusing.
Therefore continue to use 'add_drive_opts' in the examples
for now.
By using the once_had_no_optargs flag, this change is backwards
compatible for callers (except Haskell, PHP and GObject as discussed
in earlier commit).
