[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

[fifo] zx_fifo_read/write bytes->elems migration, part 5

Remove the _old compat shims

ZX-1951 #done

Change-Id: Icece05ec5d47d2eb0a73e7fcba4dbed45e866d4f

[fifo] zx_fifo_read/write bytes->entries migration, part 3

Change signatures of zx_fifo_read/write to take the number of elements
plus the element size instead of a single byte size parameter and update
the docs.

Also mark _old shims as deprecated.

ZX-1951 #comment

Change-Id: I1ad71df760e609e9540e7c332fae156d18341023

[fifo] zx_fifo_read/write bytes->entries migration, part 1

- Introduce zx_fifo_read_old and zx_fifo_write_old
- Mark zx_fifo_read and zx_fifo_write as deprecated
- Migrate all callers in zircon to the "_old" versions

ZX-1951 #comment

Change-Id: I3be0ace412a6fe9d729eaf9a581bbf88cd45effe

[kernel][syscalls] Remove redundant filename prefix

All of these files are already in a directory named syscalls.

Change-Id: If4945b107c2ae9787e63dfbfe3750d7dfdba739b