Fix-for-multiple-thread-detection-in-AIX.

Fix-for-multiple-thread-detection-in-AIX.

In AIX multiple threads were not added. This patch is a fix for the same

When we create a pthread debug session we have callbacks to read
symbols and memory.  One of those call backs is pdc_read_data.

Before we come into aix-thread wait() we switch to no thread and
therefore the current thread is null.

When we get into pdc_read_data we have a dependency that we need to
be in the correct current thread that has caused an event of new
thread, inorder to read memory.

Hence we switch to the correct thread.

This is done by passing the pid in the pthdb_user_t user_current_pid
parameter in every call back.