Fix parsing of mysql client commands, especially in relation to
single-line comments when --comments was specified.

This is a little tricky, because we need to allow single-line
comments in the middle of statements, but we don't want to allow
client commands in the middle of statements. So in
comment-preservation mode, we go ahead and send single-line
comments to the server immediately when we encounter them on their
own. 

This is still slightly flawed, in that it does not handle a
single-line comment with leading spaces, followed by a client-side
command when --comment has been enabled. But this isn't a new
problem, and it is quite an edge condition. Fixing it would require
a more extensive overall of how the mysql client parses commands.