using mbstring functions for body wrapping, if they are available.

using mbstring functions for body wrapping, if they are available.
own utf strlen implementation is removed because we need strlen, substr and
strpos implementation in order to use it correctly.
sqbodywrap function is modified to use sq_* functions instead of vanilla
string functions.
function calls that analize string in bytes ($body{$position}) are replaced
with string functions.

closes #1043576 in devel. code is not that complex in stable and it needs only
modified strlen calls.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@9146 7612ce4b-ef26-0410-bec9-ea0150e637f0