[vmplot] Fix typo in options dump
[sys_stat] simplify collection of interface stats

No need to use tricks to split only after the :, just use a different
offset!

Include some basic commentary on what things are doing
Default time column matches sys_stat logs
Move the column mappings up to the "user modifiable" section
Add lots of usage instructions to README
vmplot: explain how the timestamps are added to the graph

This was not obvious!
Add a little documentation to graphing helper
Remove "size" option to vmplot.

This makes usage simpler, as we don't have to manually run wc -l on the
file before we call vmplot.
This makes usage more reliable, as we don't have to be careful with
filenames and arguments.  (Leaving out the -s option previously meant it
ignored your specified file argument)

This DOES makes vmplot use more memory, as the entire log file is read into
memory before being written out again, instead of writing it line by
line as it was read.  Even an entire day's worth log of logging at 1 second
intervals is only 86k lines though, so this shouldn't be any problem in
practice.
Make graphs on a less custom set of filenames