perl: Don't hardcode interpreter path
    
    This is particularly useful on operating systems that don't ship
    Perl as part of the base system (eg. FreeBSD) while still working
    just as well as it did before on Linux.
    
    In one case (src/rpc/genprotocol.pl) the interpreter path was
    missing altogether.
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>