MFC r262441-262442,262447,262461-262464,262655:

- Consistently pass around context information using a simple pointer.
  This fixes some dereferencing bugs in Chinese character set conversions.
- Fix Simplified Chinese character set conversions by switching around the
  fields of an internal struct so it corresponds with the way variables of
  this type are initialised.
- Fix an array index out of bounds bug in iconv VIQR (Vietnamese) module.
- Silence gcc warning about unsigned comparison with 0.

Also record r258316 and r258587 as merged so they no longer show up as
eligible.

PR:		185964
Submitted by:	Manuel Mausz <manuel-freebsd@mausz.at>