libdrgn: debug_info: report userspace core dump debug info ourselves

There are a few reasons for this:

1. dwfl_core_file_report() crashes on elfutils 0.183-0.185. Those
   versions are still used by several distros.
2. In order to support --main-symbols and --symbols properly, we need to
   report things ourselves.
3. I'm considering moving away from libdwfl in the long term.

We provide an escape hatch for now: setting the environment variable
DRGN_USE_LIBDWFL_REPORT=1 opts out of drgn's reporting and uses
libdwfl's.

Fixes osandov#130.

Signed-off-by: Omar Sandoval <[email protected]>

Author: osandov

docs/advanced_usage.rst

@@ -100,6 +100,12 @@ Some of drgn's behavior can be modified through environment variables:
     vice versa. This environment variable is mainly intended for testing and
     may be ignored in the future.
 
+``DRGN_USE_LIBDWFL_REPORT``
+    Whether drgn should use libdwfl to find debugging information for core
+    dumps instead of its own implementation (0 or 1). The default is 0. This
+    environment variable is mainly intended as an escape hatch in case of bugs
+    in drgn's implementation and will be ignored in the future.
+
 ``DRGN_USE_LIBKDUMPFILE_FOR_ELF``
     Whether drgn should use libkdumpfile for ELF vmcores (0 or 1). The default
     is 0. This functionality will be removed in the future.