QtIFW

Let mkdmg.sh script on OS X actually create random temporary file names

When asking binarycreator to create an app bundle on OS X, temporary
files are created in /tmp via, the name of which is determined via

  DMG=`mktemp "/tmp/$VOL.XXXXXX.dmg"`

However, on OS X, this doesn't actually create a random file name. OS X
uses the BSD implementation of 'mktemp'. The man page explains:

  The trailing `Xs' are replaced with the current process number and/or
  a unique letter combination.

Note the *trailing*. Hence, on OS X, the 'XXXXXX' part should be at the
end of the template.

This manifested as a problem in case creation of the .dmg file is
aborted (e.g. via Ctrl+C) in which case the temporary file is not
removed. In that case, a subsequent attempt to create a .dmg will fail
since mktemp fails to create the temporary file - since it's already
there! The only remedy is to remove the temporary file by hand.

Alas, 'hdiutil' and 'hdid' apparently depend on the file extension in
order to detect the file format. So it's not just a matter of changing
the template for mktemp. Instead, let's pass just '$VOL.XXXXXX' to
mktemp but then tack on the file extension when using the file name.

Change-Id: I7a1a548df24054e01630c441f259031045703ff3
Task-number: QTIFW-780
Reviewed-by: Karsten Heimrich <karsten.heimrich@theqtcompany.com>