Make pmcstat provide a better warning message to users when samples are dropped

[jtl]

Summary

Change the driver stats to what they really are: unsigned values.

When pmcstat exits after some samples were dropped, give the user an
idea of how many were lost. (Granted, these are global numbers, but
they may still help quantify the scope of the loss.)

Sponsored by: Juniper Networks
Approved by: gnn (mentor) [Assuming he does... ;-)]
MFC after: 1 month

Test Plan

'make buildkernel' and 'make buildworld' succeeded.

Ran with a very low sampling frequency to create drops, and saw the new
error message.

[gnn]

Other than the code nit listed above this is ready to go into the tree. Once the formatting is fixed please commit with:

Approved by: gnn (mentor)

lib/libpmc/pmc.h

49

Does this really split a line? We don't want to do that if we can avoid it.

[jhb]

Are you worried about overflow at all? Should the messages say something like "sampling was paused at least %u times" to indicate that the problem could be much worse than indicated?

[jtl]

In D4123#86858, @jhb wrote:

Are you worried about overflow at all? Should the messages say something like "sampling was paused at least %u times" to indicate that the problem could be much worse than indicated?

Seems reasonable. I'll make the change.

[jtl]

Minor change to warning message suggested by @jhb.

This adds "at least" to the warning message to account for counter overflows.

[jtl]

lib/libpmc/pmc.h

49

Per our email conversation, I'll leave the formatting as it was in the patch in order to maintain the comment alignment between lines.

[jtl]

Committed in rS290929.