These are more efficient than separate pin/unpin calls because
pin count increment and decrement can be done cheaply when the
data pointer is known.
Secondly, pinned access can be made safe against preemption by
hardware interrupts or other CPU cores; buflib_get_data() can't.
This makes it more useful under different threading models and
for SMP targets; both of which are not particularly relevant to
Rockbox now, but might be in the future.
Change-Id: I09284251b83bbbc59ef88a494c8fda26a7f7ef26
To minimize code duplication between buflib backends move the
public part of the API to buflib.h. Also rewrote documentation
for the whole API.
Change-Id: I4d7ed6d02084d7130cb41511e63c25ec45b51703
