Using '$' is easy to misuse in shell scripts, shell commands etc. After
all this years, lets abandon this dangerous character and move to
something safer: '@'. The choice was made after reviewing specifications
of various shells on different operating systems and this is the
character that have no special meaning in none of them.
To preserve compatibility, automatically translate '$' to '@' when
loading policy files.
'dispvm_allowed' name was confusing, because it suggested being able to
spawn new DispVMs, not being a template for DispVM.
FixesQubesOS/qubes-issues#3047
