Put the LINKTYPE_ values in numerical order, to make it easier to make

Put the LINKTYPE_ values in numerical order, to make it easier to make
sure that all values from 100 to the current maximum value are in use;
put in comments to indicate for what purpose the reserved ones are
reserved.

Note in the "don't just grab a value and use it yourself" comment why
you shouldn't use values below 100 or above the maximum value you see in
the file.