Filter invalid encodings from Linux thread names

Filter invalid encodings from Linux thread names

On Linux, a thread can only be 16 bytes (including the trailing \0).
A user sent in a test case where this causes a truncated UTF-8
sequence, causing gdbserver to create invalid XML.

I went back and forth about different ways to solve this, and in the
end decided to fix it in gdbserver, with the reason being that it
seems important to generate correct XML for the <thread> response.

I am not totally sure whether the call to setlocale could have
unplanned consequences.  This is needed, though, for nl_langinfo to
return the correct result.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30618