serial_hid: add iokit= prefix for the Mac IOKit special case

serial_hid: add iokit= prefix for the Mac IOKit special case

Rephrase the logic which turns HIDAPI paths returned from enumerations
into something that can be used with conn= device options. Rearrange
code paths and rename variables to hopefully increase readability, and
to prepare support for more conditions in future implementations.

Replace the "IOService:" prefix on recent Mac versions with the "iokit="
literal, to eliminate the previously unhandled colon in path names. This
resolves bug #1586.

Move the allocation of a writable buffer from the callers to the callee,
to simplify multiple call sites, and most of all because the caller need
not be aware of the buffer's required size (input and output size can
differ in either direction).

Update the conn=hid/ section in README.devices, add the iokit= prefix.