When munging the ELF data structures from the capsule-export code path
via dl_iterate_phdr, we get the dynamic section size as a callback arg
so we can double-check we haven't ended up outside the memory region
we expect to be working on.

Unfortunately when using link_map traversal to do the same sort of thing
to install wrapper functions inside the capsule we don't have this
information. We were using SIZE_MAX on ELFCLASS64 to pass a "big enough"
size parameter but this calculation does not work on ELFCLASS32.

Rather than go out of our way to produce a bogus-but-big-enough value
we now pass 0 for the size and have the called functions know that they
can't do the double-check if passed a size of 0.

The last entry in the dyn section should have a D_TAG of DT_NULL
anyway so we'll only crash off the end of the dynamic section if
the linker is broken or malicious (which is probably not worth
defending against, all things considered).