When a Vector or QAngle rvalue reference is returned from a VScript
function, the constructed ScriptVariant_t must not store the pointer to
it since it is, per convention, a temporary reference. Only do that for
lvalue-references, but do a copy when constructing from or assigning a
rvalue reference.

Type FIELD_HSCRIPT is removed, it never sets SV_FREE.

All dynamic memory is now free`d via free(). For strdup(), this would be
the conformant way in standard C++. It matters not in Source, since both
use the same global allocator of the engine, but it is tidier.
Vector and QAngle are now allocated via malloc() instead of new to
provide symmetry.

When assigning a null value in getVariant(), make sure a previous
SV_FREE is not carried over.

In SqurrelVM::ReleaseValue(), make sure SV_FREE is not kept after
Free()ing a value.

Instead of temporary allocating dynamic memory, which subsequently is
copied into another dynamically allocated piece of memory, provide a
temporary buffer on the stack.
The script value inserted into the VM still needs to allocate
dynamically though...

A few sites are adapted to not create a ScriptVariant_t from a temporary
Vector to avoid dynamic allocation there too.

cf. mapbase-source#321

They wouldn't even work.

Free values before assignment.