[Merge] Fix bugs involving compilation and ro SVs

There was an exception in sv_force_normal_flags that allowed read-only
scalars to be modified at compile time (present since 8990e30710, aka
\perl 5.0 alpha 6), which is convenient for modifying op trees.

Unfortunately, much of the code relying on it was simply buggy.  So
this commit fixes those bugs and removes the exception.  In some cases
the bug fixes led to other bug fixes, not necessarily related to
sv_force_normal/SvREADONLY.