Support scope = "async" on parameters.
Support scope = "async" on parameters.

This flag is still useful on delegates such as ThreadFunc that are
always scope = "async" by default. The flag on the parameter will
override the flag has been set on the delegate.

flobrosch/vala-fixes,playya/vala,severinh/vala,GNOME/vala,rvu95/vala,frida/vala,Distrotech/vala,xdave/pvala,GNOME/vala,frida/vala,playya/vala,rvu95/vala,severinh/vala,flobrosch/vala-fixes,frida/vala,frida/vala,Distrotech/vala,severinh/vala,GNOME/vala,spidermonkdust/vala,playya/vala,rvu95/vala,spidermonkdust/vala,GNOME/vala,Distrotech/vala,frida/vala,spidermonkdust/vala,flobrosch/vala-fixes