Not every __sun has backtrace().

I don't know how to test for Open Solaris that has support for
backtrace() so for now removing the #ifdef that breaks compilation under
other Solaris flavors.
Fixed a bug in memtest progress bar, that had no actual effects.

This closes issue #859, thanks to @erbenmo.
Merge remote-tracking branch 'antirez/2.6' into 2.6