When a blocking system call is ongoing, the error message shown was
wrong. It was indicated that a blocking system call is on-going when
the program terminates. This is correct, but hides the error message
which describes why packetdrill is terminating.

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FreeBSD supports usleep(), so use it.
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This fixes a bug on non-Linux system for the handling of
blocking system calls. While waiting, release the state
mutex.
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The thread name is only used for debugging. On Linux this
breaks is_thread_sleeping(), since it assumes that the
process or threadname contains no space. Just don't set
this on Linux, so avoid any problems.

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This was missed in the commit addig a function for determing whether
a thread sleeps or not.

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This is required to support blocking system calls on FreeBSD.
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This helps in debugging.