email plugin: Made socket settings configurable.

email plugin: Made socket settings configurable.

Added config file support to the email plugin.

The following options are available:
* "SocketGroup <group name>"
  Set the group the UNIX socket belongs to to <group name>.
* "SocketPerms <perms>"
  Set the permissions of the UNIX socket to <perms>. No validation is done.
  The user has to make sure reasonable values are given.
* "MaxConns <conns>"
  The maximum number of concurrent connections is set to <conns>.

<perms> and <conns> may be given as decimal (no prefix), octal (prefix "0") or
sedecimal (a.k.a. hexadecimal, prefix "0x") values.

Signed-off-by: Sebastian Harl <sh@tokkee.org>