#566 Consider adding the ability to safely load invalid ELF objects

[Joseph Koshy]

Currently libelf declines to process ELF objects which are malformed. For example, elf32_getehdr() will fail if an object has an invalid ELF header.

The downside of this stance is that tools such as readelf built using our libelf will be unable to process a damaged ELF object.

Consider adding an ELF_F_INVALID flag that signals that the content of an ELF data structure is incomplete or invalid in some way, and permit libelf's APIs to return such "invalid" structures.