cfg-ext-34-4.diff:  In this patch, default values are classified as bool, int, on other.  An invalid user bool or int value is replaced by the corresponding default (which had to be a valid bool or int for the type to be set) and the invalid value is either removed or replaced by a valid value.  To this extent, this dialog will repair a corrupted config-extensions.cfg.  Int values get an entry box with validation.

The validation allows blank strings for int entries (which, of course are also allowed for 'other'.  All non-key options for default extensions must have a value.  So in set_user_value, if there is a default, a blank value is treated the same as if it were the default, and the user config line removed if there is one.  So blanking an entry is a way to set is to the default value.

I think this is about ready to commit.  It should first be tested with an added section in the user config for an added extension.  I also think set_user_value should be able to use the already fetched and saved default value.

S2: For patch 3, Rietveld shows all 5 files.  It momentarily showed 5 for patch 2, then reverted to just 1.