Speed up pmcstat by improving string hash

In one case generating callgraph output from a 24MB system-wide sampling
data file took 17.4 seconds on average.  Profiling showed pmcstat
spending a lot of time in strcmp, due to hash collisions.

Replacing the XOR-only hash with FNV-1a reduces the run time for my
test by 40%.
Add a command line argument (-l) to end event collection after some
number of seconds.  The number of seconds may be a fraction.

Submitted by:	Julien Charbon <jcharbon@versign.com>
MFC after:	2 weeks
Relnotes:	yes