[docs][syscalls] Document more syscall right requirements

ZX-2399 #done

Test: no functional change
Change-Id: Ib1dd0ff19eac9e7ceb6227e83817eeb1d0d1dbdc

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

ZX-2399 #comment

Test: no functional change
Change-Id: I1f285c677e2444fcf1d720d7ecbd9f490c303962

[docs][syscalls] Clarify ZX_ERR_NO_MEMORY as eventually going away

Not touched:
vmo_op_range.md
vmo_set_size.md
vmo_write.md
The current text is ok for these.

ZX-2303 #comment patch

Tested: Just doc changes, read before/after.

Change-Id: I2c6e1039e2b205e9cc377b9888755fa7ac676c78

[handles] Introduce zx_handle_close_many

Test: handle-close-test tests success and failure cases.

Change-Id: I85bd0fc52aad474e826a4ec8c6e3ee7f94f039b0

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[docs][errors] Rename all the errors in docs/ to MX_OK and MX_ERR_*

Change-Id: I62451456a83145760c6454a619d4c699ce8e9017

[docs] Add rights to handle_duplicate's SEE ALSO section.

Change-Id: Ic57540249c723eb01efd311b21eb53c00ee475d5

[syscalls] update mx_handle_duplicate() and mx_handle_replace()

Change-Id: Ie332f4a924311c0bf1f93031712853c33027c411

[magenta][syscalls] Add mx_handle_replace().

Basically like mx_handle_duplicate(), but doesn't duplicate (instead it
invalidates the original source handle) and doesn't require the
"duplicate" right.

This can be used to reduce the rights of a handle (but it yields a new
"replacement" handle), in particular of non-duplicatable handles.

Also fix handle-info-test:
* Actually run handle_rights_test.
* Actually fail correctly (the CHECK macro, which is now gone, returned
__LINE__ on failure)

Change-Id: I4c2724d5aa6a36ebf1e084b2ff329f8da5453804

[syscalls][api] rename _magenta_*() to mx_*()

The _magenta_ prefix is pretty unwieldy.
This change simplifies that a bit.

This was done mechanically, and boots, runs, and passes tests
on qemu arm64, amd64, and NUC.

Change-Id: I8eb07db6ed6f4b8cac96344cc4ad65ce3f85f917

[docs] move magenta docs up a level, make them more front and center

(and make the paths shorter for doc urls, editing, etc)

Change-Id: I1416d8e93aa8732e2735027e6356d629adebea41