client: Fix rogue execute protection

client: Fix rogue execute protection

To protect against a rogue server reading your files, the client
honors C_EXECUTE only when it matches recent player input.

This has a somewhat troubled history, detailed in the previous commit.

The remaining major issue comes from commit 8b7d0b9 (v4.3.11): any
suffix of a recent line of input is accepted as C_EXECUTE text.
Before, only text that looked like an argument of an execute command
or a redirection was accepted.

Fix by again requiring the text to be preceded by something that looks
like an execute command.  But do it more carefully: don't break
execute with a prompted for argument, and prevent abuse of
redirections for execute.

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