MINOR: sym: collect the symbols known to the dynamic linker

MINOR: sym: collect the symbols known to the dynamic linker

The dynamic linker keeps .dynsym/.dynstr of every loaded object mapped for
relocation. Where dl_iterate_phdr() exists, walk each object's PT_DYNAMIC
to collect its exported function symbols from the memory image, plus its
address range from PT_LOAD. This works even when files are unreadable (e.g.
after chroot) and gives what dladdr() sees, but lock-free.

When an address falls past the nearest lower symbol, resolve_sym_name() now
prints "sym+off/size" instead of a bare offset, easier to feed to gdb. Add
"tune.disable-elf-symbols" to restore the previous output.