FEAT: ELF: emit DLLs as objects with ET_DYN type
FIX: ELF: rename word used for imported symbols for clarity

Renamed from 'symbols' to 'imports', to prepare for also supporting
exported symbols.
FEAT: ELF: use 0 as base address for DLLs
FEAT: ELF: basic export support

Exporting functions should work. Exporting global variables needs
further work. on-load/on-unload is not yet implemented.