Add 'minlen' option for topo graphs

Running build_topo_graph.pl with -m 2 increases the spacing between the
nodes and makes it less likely for the interface names to overlap.

Fixes redmine #1821 if you update your topo crontab entries to add -n -m 2
Add line length setting in device topo view

Makes it easier to see interface names if "Show Interface Names?" has
been selected
Options for rebuilding whole topo graphs

Add options 'Show Interface Names' and 'Line Length' when rebuilding the
whole topo graphs in the GUI

Fixes redmine #1821
Swap head/tail on "up" links

Previous change for #1824 ended up with head/tail labels on the wrong ends
fix for 1765