libelf: Use a red-black tree to manage the section list.

The tree is indexed by section number. This speeds up elf_getscn()
and its callers, which previously had to traverse a linked list. In
particular, since .shstrtab is often the last section in a file,
elf_strptr() would have to traverse the entire list.

PR: 234949
Reviewed by: emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20443

libelf: reload section headers after update with ELF_C_WRITE

These issues (PR 218860 and PR 218861) are still undergoing additional
discussion in the upstream context and there may be additional changes
to come.

PR: 218860
Submitted by: Eric McCorkle
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D10486

libelf: add an assert that msz is non-zero

Reported by: Coverity
CID: 976023