Bug 6752: Be stricter with utf-8 encoding of output

Bug 6752: Be stricter with utf-8 encoding of output

use encoding(UTF-8) rather than utf-8 for stricter
encoding
Marking output as ':utf8' only flags the data as utf8
using :encoding(UTF-8) also checks it as valid utf-8
see binmode in perlfunc for more details
In accordance with the robustness principle input
filehandles have not been changed as code may make
the undocumented assumption that invalid utf-8 is present
in the imput
Fixes errors reported by t/00-testcritic.t
Where feasable some filehandles have been made lexical rather than
reusing global filehandle vars

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>