Complete rewrite of vlc_symbols.h generation (closes #155)

Complete rewrite of vlc_symbols.h generation (closes #155)

New VLC_EXPORTs are now added at the end of module_symbols_t so that
offsets of other function pointers are preserved.

Deleted VLC_EXPORTs are replaced with dummy pointers for padding.

Of course, it doesn't solve the problem with changing shared structures
format :/ and it may cause SVN conflicts when multiple people changes
VLC_EXPORT. I doubt it can be avoided though.