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 redox-os/relibc!475

Cleanup ld_so organization and move pthread_t into TCB

See merge request redox-os/relibc!474

Various futex fixes

See merge request redox-os/relibc!470

Fix double slashes in fpath.

See merge request !469

Add tests for unistd.h constants

See merge request !467

Protect fork() and clone() behind a rwlock.

See merge request !468

rlct_clone will acquire a read lock, whereas fork will acquire a write
lock. The write lock is necessary because the fork will clone the file
table, which would result in other threads' fork/clone file descriptors
not being closed. If an address space switch fd never gets closed, fork
child processes and new threads, may never switch address spaces before
they are started, which has resulted in hard-to-debug RIP=0 instr fetch
page faults.

Fix pthread rwlock code

See merge request redox-os/relibc!466

Don't map thread stacks as MAP_SHARED (lol), fix i686 signal trampoline

See merge request redox-os/relibc!465

MAP_SHARED means it will continue to be shared after forks, which
obviously isn't the correct behavior. `acid thread` doesn't segfault
infinitely, anymore.

Implement the event queue ABI.

See merge request redox-os/relibc!454