Four distinct fixes for significant incorrectness bugs, as three small commits

See merge request redox-os/relibc!481

Without this fix, `tests/bins_static/pthread/rwlock_randtest` would sometimes get stuck blocking
forever with only 1+ threads all doing `pthread_rwlock_wrlock()` but all the other threads
having finished.

Improve the testing instructions on the README

See merge request redox-os/relibc!477

Document the test instructions and improve the title

See merge request redox-os/relibc!476

Implement pread and pwrite using the syscalls.

See merge request !475

Cleanup ld_so organization and move pthread_t into TCB

See merge request !474