Add file links_and_mods.cif

First step in splitting link descriptions out of list/mon_lib_list.cif.
For now, the link description is just duplicated:

    sed -n '/^data_link_list/,$p' list/mon_lib_list.cif > links_and_mods.cif

When we have major programs reading links_and_mods.cif,
this part can be deleted from mon_lib_list.cif.

The goal is to make reading of links links faster (in particular, that's
important when reading from a web server). The size of links_and_mods.cif
is only 8% of mon_lib_list.cif (312406 vs 3990585 bytes).