The callback parameter of the RPC interface currently allows for
specifying a prefix of arbitrary length of the returned result. This can
be exploited by certain attacks.

As a countermeasure, this patch restricts the allowed character set for
the callback name to letters, digits, underscores, parenthesis and dots.
It also limits the length of the name to 128 characters. Furthermore,
the reflected callback name is now always prepended with "/**/", which
is a common workaround to protect against attacks such as Rosetta Flash.

Fixes FS#46259.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>