MFC r328307, r328311-r328312

r328307:
Fix a bug introduced with recursive bus ownership support in r321584.

The recursive ownership support added in r321584 was unconditionally in
effect all the time -- whenever a given i2c slave device instance tried to
lock the i2c bus for exclusive use when it already owned the bus, the call
returned immediately without waiting. However, many i2c slave drivers use
bus ownership to enforce that only a single thread at a time can be using
the slave device. The recursive locking changes broke this use case.

Now there is a new flag, IIC_RECURSIVE, which can be mixed in with the
other flags passed to iicbus_acquire_bus() to allow drivers to indicate
when recursive locking is desired. Using the flag implies that the driver
is managing concurrent access to the device by different threads in some way.

This immediately fixes all existing i2c slave drivers except for the two
i2c RTC drivers which use the recursive locking feature; those will be
fixed in a followup commit.

r328311 and r328312:
Follow changes in r328307 by using new IIC_RECURSIVE flag.

The driver now ensures only one thread at a time is running in the API
functions (clock_gettime() and clock_settime()) by specifically requesting
ownership of the i2c bus without using IIC_RECURSIVE, then it does all IO
using IIC_RECURSIVE so that each individual IO operation doesn't try to
re-acquire the bus.

The other IO done by the driver happens at attach or intr_config_hooks time,
when there can't be multiple threads running with the same device instance.
So, the IIC_RECURSIVE flag can be safely ORed into the wait flags for all IO
done by the driver, because it's all either done in a single-threaded
environment, or protected within a block bounded by explict
iicbus_acquire_bus() and iicbus_release_bus() calls.