Fix hwpmc "stalled" behavior

Currently, there is a single pm_stalled flag that tracks whether a
performance monitor was "stalled" due to insufficent ring buffer
space for samples. However, because the same performance monitor
can run on multiple processes or threads at the same time, a single
pm_stalled flag that impacts them all seems insufficient.

In particular, you can hit corner cases where the code fails to stop
performance monitors during a context switch out, because it thinks
the performance monitor is already stopped. However, in reality,
it may be that only the monitor running on a different CPU was stalled.

This patch attempts to fix that behavior by tracking on a per-CPU basis
whether a PM desires to run and whether it is "stalled". This lets the
code make better decisions about when to stop PMs and when to try to
restart them. Ideally, we should avoid the case where the code fails
to stop a PM during a context switch out.

Optimizations to the way hwpmc gathers user callchains

Changes to the code to gather user stacks:
* Delay setting pmc_cpumask until we actually have the stack.
* When recording user stack traces, only walk the portion of the ring
  that should have samples for us.

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.)

Improve accuracy of PMC sampling frequency

The code tracks a counter which is the number of events until the next
sample. On context switch in, it loads the saved counter. On context
switch out, it tries to calculate a new saved counter.

Problems:

1. The saved counter was shared by all threads in a process. However, this
means that all threads would be initially loaded with the same saved
counter. However, that could result in sampling more often than once every
X number of events.

2. The calculation to determine a new saved counter was backwards. It
added when it should have subtracted, and subtracted when it should have
added. Assume a single-threaded process with a reload count of 1000
events. Assuming the counter on context switch in was 100 and the counter
on context switch out was 50 (meaning the thread has "consumed" 50 more
events), the code would calculate a new saved counter of 150 (instead of
the proper 50).

Fix:

1. As soon as the saved counter is used to initialize a monitor for a
thread on context switch in, set the saved counter to the reload count.
That way, subsequent threads to use the saved counter will get the full
reload count, assuring we sample at least once every X number of events
(across all threads).

2. Change the calculation of the saved counter. Due to the change to the
saved counter in #1, we simply need to add (modulo the reload count) the
remaining counter time we retrieve from the CPU when a thread is context
switched out.

Support a wider history counter in pmcstat(8) gmon output

pmcstat(8) contains an option to output sampling data in a gmon format
compatible with gprof(1). Currently, it uses the default histcounter,
which is an (unsigned short). With large sets of sampling data, it
is possible to overflow the maximum value provided by an (unsigned
short).

This change adds the -e argument to pmcstat. If -e and -g are both
specified, pmcstat will use a histcounter type of uint64_t.

Fix the date on the pmcstat(8) man page from r291016.