threads: fix unitialized members in sched_param
threads: fix unitialized members in sched_param

Building with gcc 8.2 against musl libc, which apparently has more attributes
available in its sched_param. The following warnings were produced:

    warning: missing initializer for member 'sched_param::sched_ss_low_priority' [-Wmissing-field-initializers]
    warning: missing initializer for member 'sched_param::sched_ss_repl_period' [-Wmissing-field-initializers]
    warning: missing initializer for member 'sched_param::sched_ss_init_budget' [-Wmissing-field-initializers]
    warning: missing initializer for member 'sched_param::sched_ss_max_repl' [-Wmissing-field-initializers]

Since the current thread may have interesting non-zero values for these fields,
we want to be sure to only change the intended one. Query and modify the
current sched_param rather than starting from a zeroed one.

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