Introduce the double-diamond operator <<>>

This operator works like <> or <ARGV>, as it reads the list of file
names to open from the command-line arguments. However, it disables
the magic-open feature (that forks to execute piped commands) :

    $ bleadperl -e 'while(<>){print}' 'echo foo |'
    foo
    $ bleadperl -e 'while(<<>>){print}' 'echo foo |'
    Can't open echo foo |: No such file or directory at -e line 1.
Added some documentation for while(<<>>)
Add tests for the <<>> operator
Add tests for $ARGV

There weren't apparently any. This also tests that $ARGV behaves
correctly both with <> and <<>>.
Clarify the documentation for <<>>
Add tests for empty strings in @ARGV

which should be refused, since no file name should be empty.
(Suggested by Richard Soderberg)