libzpool: set thread names

Arrange for the thread/task name to be set when new threads are created.
This makes them visible in the process table etc.

pthread_setname_np() is generally available in glibc, musl and FreeBSD,
so no test is required.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16140