After the initial discussion from #3163, and looking more closely at how
variable access is handled in subroutines I noticed a discrepancy.

Setting a read only variable like obj.ttl in vcl_recv would result in
a misleading error message explaining that it is read only, instead of
simply not available.

This change defers the illegal write check, registering unconditionally
that the symbol was used in a set action. As a result we always get the
correct error message but depending on whether this is happening in a
vcl_ or custom subroutine we may either get "in subroutine" or "from
subroutine" in the error message. A minor discrepancy probably worth
getting rid of the prior inconsistency.

This is covered by the v21 test case.