The utilities in os/session_policy.h used to be tailored for the
matching of session arguments against a server-side policy
configuration. However, the policy-matching part is useful in other
situations, too. This patch removes the tight coupling with the
session-argument parsing (via Arg_string) and the hard-wired use of
'Genode::config()'.
To make the utilities more versatile, the 'Session_label' has become a
'Genode::String' (at the time when we originally introduced the
'Session_label', there was no 'Genode::String'). The parsing of the
session arguments happens in the constructor of this special 'String'.
The constructor of 'Session_policy' now takes a 'Genode::String' as
argument. So it can be used with the 'Session_label' but also with other
'String' types. Furthermore, the implicit use of 'Genode::config()' can
be overridden by explicitly specifying the config node as an argument.
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