Add option for new max_children per operation

[pablodav]

references:
grke/burp#566
http://burp.grke.org/docs/manpage.html

Since version 2.1.10

• Add the ability for the client to connect to different server ports
  according to whether it is doing backup/restore/verify/list/delete.

On server:

Add multiple max_children or max_status_children

       port=[port number]
	      Defines  the  main  TCP port that the server listens on. Specify
	      multiple 'port' entries on separate lines in order to listen  on
	      multiple	ports.	Each  port  can  be  configured  with  its own
	      'max_children' value.

So result will be:

# Port for backups
port=
max_children

On client:

       port=[port number]
	      Defines  the  TCP  port on the server that we will
	      send requests to. If this option is set, it is the
	      default for these options, which can be overridden
	      individually: port_backup, port_restore, port_ver‐
	      ify, port_list, port_delete. If this option is not
	      set, you will need to set all of the port  options
	      separately.

       port_backup=[port number]
	      Defines  the  TCP  port on the server that we will
	      send backup requests to. If not set,  it	defaults
	      to the port option.

       port_restore=[port number]
	      Defines  the  TCP  port on the server that we will
	      send restore requests to. If not set, it	defaults
	      to the port option.

       port_verify=[port number]
	      Defines  the  TCP  port on the server that we will
	      send verify requests to. If not set,  it	defaults
	      to the port_restore option.

       port_list=[port number]
	      Defines  the  TCP  port on the server that we will
	      send list requests to. If not set, it defaults  to
	      the port option.

       port_delete=[port number]
	      Defines  the  TCP  port on the server that we will
	      send delete requests to. If not set,  it	defaults
	      to the port option.

       status_port=[port number]
	      Defines  the TCP port that the server is listening
	      on for status requests.