Use mozjpeg defaults by default

Since mozjpeg is now backward ABI-compatible with libjpeg[-turbo], it is now
possible to temporarily load mozjpeg into a binary application and cause that
application to generate uber-compressed JPEGs (at the expense of an extreme
performance loss, of course.)  For instance, someone could do

LD_LIBRARY_PATH=/opt/mozjpeg/lib convert blah_blah_blah

to make ImageMagick use mozjpeg instead of the system's pre-installed JPEG
library (libjpeg-turbo, in most cases.)  However, this only makes sense if
mozjpeg is actually producing different behavior by default than libjpeg-turbo.
Currently it isn't.  Currently it requires the application to set
JBOOLEAN_USE_MOZ_DEFAULTS to TRUE in order to enable the mozjpeg-specific
behavior, but of course applications that were built to use libjpeg[-turbo]
won't do that.  Thus, this patch sets use_moz_defaults to TRUE by default,
requiring an application to explicitly set it to FALSE in order to revert to
the libjpeg[-turbo] behavior (makes sense, since the only applications that
would need to revert to the libjpeg[-turbo] behavior would be mozjpeg-aware
applications.)

Note that we discussed the possibility of adding a function
(jpeg_revert_defaults()), which would act the same as jpeg_set_defaults() does
in libjpeg[-turbo].  This is a good solution for implementing the -revert
switch in cjpeg, but unfortunately it doesn't work for jpegtran.  The reason
is that jpeg_set_defaults() is called within the body of
jpeg_copy_critical_parameters(), which is part of the API.  So yet again,
if mozjpeg were loaded into a non-mozjpeg-aware application at run time, it
would be desirable for jpeg_copy_critical_parameters() to set the parameters
to mozjpeg defaults.  That means that, in order to implement the -revert
switch in jpegtran, it would be necessary to introduce a new function
(jpeg_revert_critical_parameters(), perhaps).  It seems cleaner to just keep
using the JBOOLEAN_USE_MOZ_DEFAULTS parameter to control the behavior of
jpeg_set_defaults(), even though this represents a minor abuse of the libjpeg
API (jpeg_set_defaults() is technically supposed to set all of the parameters
to defaults, irrespective of any previous state.  However, as long as we
document that JBOOLEAN_USE_MOZ_DEFAULTS works differently, then it should be
OK.)