Switch cache-long-scans to t by default.

Switch cache-long-scans to t by default.

 src/xdisp.c (message_dolog): Make sure the *Messages* buffer has its
 cache-long-scans disabled, since we don't want to call
 prepare_to_modify_buffer (in insert_1_both) for each message we
 display.
 src/buffer.h (bset_cache_long_scans): New INLINE function, moved
 from buffer.c.  Improve commentary to the buffer field setter
 functions.
 src/buffer.c (bset_cache_long_scans): Static function deleted.
 Improve commentary to the buffer field setter functions.
 (init_buffer_once): Default for cache-long-scans changed to t.
 (Bug#15797)