RFC: Rename neosmart_event_t to pevent_t

[mqudsi]

I'm opening this as a request for comments in case anyone has any feedback before squash-merging the commits in this PR into the master branch.

The rationale for using pevent_t instead of the previous neosmart_event_t or the other option of pthread_event_t (which is probably the "classiest" option) is described in the commit messages:

The original name was an attempt to keep with the naming conventions
used by the pthreads library, along the lines of libname_objname_t.
However, it doesn't seem necessary or proper to do so if we are using
C++ with the code in the neosmart namespace.

It also doesn't seem correct to use pthread_event_t as the name if the
event object is not part of the pthread library proper, even if it is
built atop of it. Furthermore, there's no need for the name to be so
long and wordy if we have the advantage of C++ namespacing, since
there's no risk of a name collision.