[fdio] Use -fvisibility=hidden

This exports only the symbols declared in the public headers.

ZX-1895 #comment

Test: full system build
Change-Id: Idf762ea94518d668c26235047a5917bc4d4399ef

[fdio] Reduce usage of 'misc' operation.

Rather than relying on an ioctl-like 'misc' function
which acted as a bottleneck for both remote and
socket operations, split operations into separate
optional function pointers.

This change provides type safety to remote operations,
and makes it less cumbersome to add / modify these
ops.

Additionlly, fix an fd leak in futimens.

Test: Expanded fdio tests, fs tests

Change-Id: I78f0c2a4291636b35727a9bcbfeaa4e67fdcbce1

[logging] Migrate zircon off zx_log_read and zx_log_write

zx_log_create is trickier. It's essentially a version of
zx_debuglog_create that allows not passing a root resource.

ZX-2184

Change-Id: Ida3d6bacce2c4601a4c6a00fd0cf13c902eb1d41

[fdio] Move headers into lib/fdio/...

Change-Id: Ie8d74e716da913bf6e2672c4acf8cd67b4962b7f

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94