Deprecate the OStask_ID field within the property structure.

Replace with a new API to perform a task ID reverse-lookup
based on an abstract data object (the system task ID).

Resize buffers and tweak string copies to avoid truncation warnings
in GCC 9.  Note the code was generally OK (handled truncation) but
the new compiler still generates warnings even with correct usage
of certain functions.

In the UT_os_printf macro, be sure to "use" the return value
of the snprintf() call, which avoids warnings about truncation.

Fix #437, Add new API for obtaining exception task ID

Fix #435, fix string manipulations to avoid warnings

Tidy up readme markdown