More improvements/adjustments to interface flags/variables handling based
on conversion of a couple of drivers:

• Rename if_getflags() -> if_get(), if_setflags() -> if_set(). The function itself deal not only with flags, but with any integer values.
• Reimplement flag manipulation macros as inlines.
• Reimplement if_getsoftc() over if_getfeature().
• Provide access to IF_INDEX, IF_FLAGS, IF_FIB, IF_LLADDR, IF_BPF, IF_NAME.
• Provide typecasting inlines for if_name(), if_lladdr() working over if_getsoftc().
• Remove if_capenable(), instead automatically callbacks upon modifications of certain values in if_set_special().

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