m_config: optimize initialization of each option

Options with dynamic memory allocations (such as strings) require some
care. They need to be fully copied on initialization, and if a static
default value was declared, we must not free that value either.

Instead of going through the entire thing even for simple types like
integers, really run it only for options with dynamic allocations. To
distinguish types which use dynamic allocations, we can use the fact
that they require a free callback (otherwise they would leak). As a
result initialization of simple types becomes chaper, and the init
function does nothing at all if src==dst for a simple type.

(It's funny how mplayer had M_OPT_TYPE_DYNAMIC since 2002, until we
replaced it by the same heuristic as used here in commit 3bb134969eb6.
It's also funny how the new check was used only for some asserts, and
finally removed in commit 7539928c1c. I guess at this time I felt like
having uniform code was more important than pointless
micro-optimizations.)

The src==NULL case is removed because it can't happen.

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