debuginfod 1/2: client side

debuginfod 1/2: client side

Introduce the debuginfod/ subdirectory, containing the client for a
new debuginfo-over-http service, in shared-library and command-line
forms.  Two functions in libdwfl make calls into the client library to
fetch elf/dwarf files by buildid, as a fallback.  Instead of normal
dynamic linking (thus pulling in a variety of curl dependencies),
the libdwfl hooks use dlopen/dlsym.  Server & tests coming in patch 2.

Signed-off-by: Aaron Merey <amerey@redhat.com>
Signed-off-by: Frank Ch. Eigler <fche@redhat.com>