libelf: Use a red-black tree to manage sections

[markj]

Summary

The tree is indexed by section number. This speeds up elf_getscn(3),
which in turn helps other functions. For example, elf_strptr() needs to
look up .shstrtab, which is typically the last section.

[markj]

• Remove unintentional change.

[emaste]

sys/tree.h works on all supported operating systems? (for upstreaming)

Overall LGTM

[markj]

In D20443#443056, @emaste wrote:

sys/tree.h works on all supported operating systems? (for upstreaming)

Yes, it comes with libbsd, which is already required for queue.h etc..