add support for client linked with libpthread but not creating threads

We document that we do not support clients linking with libpthread, due to the loader and TLS interactions as well as transparency issues of creating new threads. This issue covers trying to add support for a client linking with libpthread but not actually creating new threads, which seems to come up a fair amount:
https://github.com/DynamoRIO/dynamorio/issues/956#issuecomment-64847764, https://groups.google.com/forum/#!topic/dynamorio-users/BKfQKybcthU, https://groups.google.com/forum/#!topic/dynamorio-users/ssPUb8cC49Q, https://groups.google.com/forum/#!topic/dynamorio-users/ED8nbG2tIdU.

The issue names no files, tests, or entry points. Start by reviewing the documented libpthread limitation and the linked issue and user discussions, then establish how support for clients that do not create threads should be verified across loader, TLS, and transparency behavior.

Written by the indexing model from the issue text.