bandwidthd add enable option

This adds a checkbox to enable or disable bandwidthd. Note:
a) For existing configs that do not have the <enable> field in config.xml, it will startup bandwidthd anyway, as per previous behaviour. Next time the user saves the bandwidthd settings, they will need to check the "Enable" box. Then the <enable> field will be in config.xml...
b) When enable is off, all the parameter settings can still be entered, so the user can set all the required parameters ready to go, but not actually start bandwidthd.
c) When the user turns off "enable", the current graphs, output log files etc are left in place on the disk (for nanobsd "disk" is a memory disk /var). They will be there ready to go if the user enables bandwidthd again, or the user can copy them elsewhere or whatever... They are not lost immediately.
I plan to do another change soon to add the option to auto-save the output log files on nanobsd - hope to get around to it this week. I will make a version change with that.