ELF unexec: Don't insert a new section

ELF unexec: Don't insert a new section

Reuse the .bss section instead, making it SHT_PROGBITS.  This way we
don't need to mess with symbol st_shndx, or section sh_link and
sh_info.

This does lead to eu-elflint complaints about symbols defined in .bss
with a needed version, because normally it is undefined symbols that
have needed versions;  Defined symbols have version definitions.
The exception is symbols defined by the linker in .dynbss for
variables copied from a shared library in order to avoid text
relocations, with copy relocs to copy their initial values from the
shared library.  These symbols are both defined and have needed
versions, and eu-elflink only expects to see them in SHT_NOBITS
sections.  Of course there is no real problem with having such symbols
in SHT_PROGBITS sections.  glibc ld.so handles them fine.

* unexelf.c: Delete outdated comments.
(PATCH_INDEX): Delete.
(find_section): Delete.
(unexec): Don't add a new section.  Instead reuse the last bss
section, extending it to cover dumped data.  Make bss sections
SHT_PROGBITS.  Remove all patching of sh_link, sh_info and
st_shndx.  Rename bss sections.