Also, force rewriting of code signature padding at every update so
that we take into account possible section relocs and expansion of
the last preceeding section, e.g., the string table.

This commit also tweak the logic responsible for managing debug lines
in `DebugSymbols`. In particular, in case we update the same function,
we'd previously incorrectly create a cycle adding pointer to the same
`SrcFn` to itself.