Fix more XS problems in Encode.xs file

[pali]

• Remove 'eval require Encode::MIME::Name' from Encode::XS code
• Fix memory leak in Encode::XS::mime_name()
• Fix function Encode::XS::renew to not alias input and output value
• Use boolSV instead of newSViv in functions Encode::_utf8_off and Encode::_utf8_on
• Use return value of eval_pv instead of $@ in function Encode::XS::perlio_ok
• Disable prototypes for Encode::XS object methods
• With CODE: directive use void return value only for subroutines which do not return a value and do not set ST(0) directly
• Use PREINIT: directive for variable declarations and INIT: for initial assignment

[dankogai]

Thank you!