extra-events column names could use improvement

[travisdowns]

When the user specifies --extra-events=... we need names for the column headers displaying those names. Currently we just take the first 6 characters of the event name, which sometimes isn't meaningful and often leads to duplicates when you have several related events all with the same first six characters.

We should do better. Perhaps smartly generate the name by taking characters from after the first or last underscore. This could even be dynamic: i.e,. check that no abbreviations end as duplicates based on the selected events and if there are, change the name generation so that they aren't.

Alternatives are possible, such as just calling the events EVT1, EVT2, etc, or adding more header rows to allow more characters, etc.

[travisdowns]

You can now add column names by putting #name after each event definition, like this:

--extra-events=uops_dispatched_port.port_0#p0,uops_dispatched_port.port_1#p1,uops_dispatched_port.port_2#p2

This will give column names p0, p1 and p2.