[docs][syscalls] Add TODOs about documenting system call rights

ZX-2399 #comment

Test: no functional change
Change-Id: I1f285c677e2444fcf1d720d7ecbd9f490c303962

[syscalls] Update parameter types and names

This CL updates the parameter types and names for a number of syscalls
to match the Zircon System API rubric. There's more work to do to make
all the syscalls conform to the rubric, but this change is a start.

Change-Id: I218ac5e7e0cbd80a8c69fef7891ad40b78b6b407

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[ulib][magenta][runtime][musl] Don't use kernel resources for dead threads

A joinable thread that has exited does not need to keep a kernel
thread object alive via its thread handle. Only the memory for
the thread descriptor needs to stick around. By using a futex
rather than an object signal, joins of threads that have already
exited can complete with no system calls.

Change-Id: I2e3eec95e2cf6897ecd3ce31205bc6b018ad40c9

[ulib][magenta][musl] Stop leaking detached threads' descriptor blocks

This adds a three-in-one vDSO call to roll together mx_vmar_unmap,
mx_handle_close, and mx_thread_exit so a thread can deallocate the
very same stack it's using to call into the vDSO. With this,
detached threads exiting can do their last bit of cleanup so there
are no leaks and no other thread is required to do it later.

MG-549 #done

Change-Id: I4c872aa847582aa18042b8fca3c22f26f6e52e2a