[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[utest] Remove empty-string msg args from ASSERT_*() calls in C++

The msg argument to ASSERT_*() is optional in C++ now, so we can drop
this argument when it's an empty string.

MG-905

Change-Id: I6a5d6917518f8c9c4865b761dcbefe296d54c924

[fs][rio] Implement FCNTL (GETFL, SETFL)

MG-854 #done

Change-Id: I8c3317860427a40917ce4f2e7e8890fdf1ecff4d