This changes podwrapper so that the input (POD) files should not
contain an =encoding directive. However they must be UTF-8.
Podwrapper then adds the '=encoding utf8' directive back during final
generation.
This in particular avoids problems with nested =encoding directives in
fragments. These break POD, and are undesirable anyway.
The previous translation from Perl slavishly followed the Perl code a
bit too much and used a named pipe to communicate between the
uncompressing subprocess and libguestfs tar-in. From C we can use an
anonymous pipe instead.
This updates commit d3512deb67.
