www/url_handlers.c: Do POST then GET in prefs_fmap()
www/url_handlers.c: Do POST then GET in prefs_fmap()

When updating the field mappings @ /prefs/fmap/ do a GET after the
POST, this avoids problems with double form submissions and makes
the browsers back button do the right thing.

See: http://en.wikipedia.org/wiki/Post/Redirect/Get

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@opentechlabs.co.uk>

ac000/receiptomatic