session_policy_local: Rework policy file handling

The old assumption was that a config file is associtated
with one session only. With introducing UID/GID support a policy
might be used for several sessions. Furthermore, it was assumed
that the file name is the key/ident to identify a session and
a file containts exactly one policy.

Here are the new rules for writing a policy file.

- A valid file name contains letters or numbers and must have a '.policy'
  suffix.
- The file name has not semantical meaning
- A policy file may contain contain more than 1 policy
- Each policy entry starts with 'policy_'
- Each policy entry shall have one and exactly one valid key (e.g.
  selinux)

The implementation has two main hash tables, file_hash and session_hash
which owns 'the file' respectively the session config. Additionally there
is a hash table which connects a policy with a session (selinux_hash).