client: Improve the client's messages

client: Improve the client's messages

Use a "Warning: " prefix for server output violating the protocol and
for rogue redirections and executes.  Don't shout "WARNING!"

In redir_authorized(), check for server issues (conflicting
redirections, rogue redirections and executes) before enforcing
restrictions (restricted mode, executing batch file), so server issues
aren't masked.

Surprisingly, popen() may not set errno on failure.  Avoid reporting a
bogus errno in dopipe().

doexecute() complains about an "execute file".  We call that a "batch
file" elsewhere.  Reword for consistency.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>