fix Issue 10125 Unicode non-BMP decoding to dchar in stdio readln

strings were first decoded to wchars, each wchar was then separately
decoded to dchar, resulting in 2 dchars in the surrogate block instead
of 1 correct dchar.

added unit test to verify readln decoding of non-ASCII characters
combine some statements
Merge pull request #1296 from M-frankied/readln-nonbmp

fix Issue 10125 Unicode non-BMP decoding to dchar in stdio readln

Merged.