Race while registering pre-shutdown functions

[K-ballo]

Occasionally test fail because the runtime attempts to register a pre-shutdown function after the shutdown process has already started. For example, see http://hermione.cct.lsu.edu/builders/hpx_gcc46_x8664_boost150_debug/builds/730/steps/run_regression_tests/logs/stdio

{what}: Too late to register a new pre-shutdown function.: HPX(invalid_status)

[K-ballo]

Seen again, on a different test: http://hermione.cct.lsu.edu/builders/hpx_gcc44_x8664_boost147_debug/builds/703/steps/run_unit_tests/logs/stdio

[hkaiser]

Let's watch this for a while. I committed possible fix for this here: 9e26c9e

[hkaiser]

This has not occurred again since the change. Closing...