Various futex fixes

See merge request !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 !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 !454

Implement new low-level signal trampoline

See merge request !460

Fix thread spawning on aarch64

See merge request !463

* Correctly align the stack
* Fix argument order of the ldp instructions
* Remove unnecessary ldr x5 instruction

Add tests for further sys/resource.h constants

See merge request !462

Add stub for SIOCATMARK command of ioctl.

See merge request !461

Fix posix prio process (same as MR458 with fixed commits)

See merge request !459

Use standard casing and mangling for internal errno

See merge request redox-os/relibc!457

Add Crypt functions

See merge request redox-os/relibc!453

Use Cell representation for errno

See merge request redox-os/relibc!456