add support for per-instance exit policies

[nusenu]

The is implemented using variable tor_ExitPolicy_file which points to a semicolon separated csv file.

The csv file contains the instance identifier in the first column followed by the ExitPolicy

Example:

myrelay-192.168.1.1_443;reject *:25,reject *:123

If an instance is not specified in the file the variable tor_ExitPolicy is taken into account, so you can specify a general exit policy and use tor_ExitPolicy_file to override specific exit policies.