Fixes unicode support

[katzenbaer]

Some libssh methods that take a char pointer and char count need to use String.utf8.count instead of String.count, else the parameter will be malformed.

  1> "你好".count
$R0: Int = 2
  2> "你好".utf8.count
$R1: Int = 6

[Lakr233]

Found this issue years ago, and you fixed it. My hero!