pmxcfs: add verify_token IPC request

pmxcfs: add verify_token IPC request

Add a new IPC request which takes a token string and matches it with
the priv/token.cfg shadow file, this allows non-root processes with
the privilege of doing IPC requests, to verify tokens without being
able to read the full token list itself.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
[ Thomas: solved merge conflict in observer files struct ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>