Merge branch 'master' of https://github.com/hpctoolkit/hpctoolkit

add description that blocktime event is only for 4.3 or newer

In some kernels, although the version is older than 4.3, but their perf_event.h provide attribute context_switch which is in theory only available with kernel 4.3 or newer.
If this is the case, the configure will think the current platform is the new one, and "activate" kernel blocking module (BLOCTIME event). There is no build error nor crash during execution time. Only undefined event.

add discussion of known issues in release notes.

Add dyninst 9.3.0 and libdwarf 2016-11-24 to the release notes.

another set of cleanup edits for environment variables in the manual.