Fix client redirection and execute for non-ASCII characters

Fix client redirection and execute for non-ASCII characters

The client rejects unauthorized redirection and execute.  Its
authorized check always fails for arguments with non-ASCII characters.
The culprit is ring_search(): It compares plain char to unsigned char,
which breaks when char is signed.  Believed to be broken in commit
8b7d0b91, v4.3.11.

Note that non-ASCII characters only work in UTF-8 sessions.  In ASCII
sessions, the server replaces them, and the authorized check fails.
Works as designed.