Program Name should come from the symbols table, not the ELF section

[dave-tucker]

Since libbpf 1.0, section names got strict (actually this happened in an earlier point release).

aya-bpf sometimes includes the program name as part of the section name... meaning libbpf will now flat out refuse to load programs that used aya-bpf in that manner. We also presented an inconsistent UX to users...
Sometimes we use the section name as default - e.g xdp - other times we use the actual function name.

In order to be libbpf 1.0 compatible - and generally to make aya easier to use - we should:

1. Fix the aya-bpf macros to be consistent - we can continue to support name but it will be become shorthand for export_name
2. Use the ELF symbol table to find multiple programs in a the same ELF section.
3. Document that in the Bpf::program and Bpf::program_mut APIs that programs are uniquely identified by their function name