file_loadraw() can have a need to load unverified files
like "dtrace_dof". Allow severity_guess() to look at the filename
but if it returns less than VE_MUST check that the type is not
one we insist on verifying.

In vectx_open if severity < VE_MUST we can allow it to be unverified.
If passed VE_GUESS we call severity_guess().
Regardless, we record severity in ctx so it is available to vectx_close().

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