When available, use /proc/$pid/map_files to read build-ids

This allows us to access binaries that are actually loaded into
a process. Reading build-ids from libraries and executables in the
filesystem is not reliable as those can be deleted or replaced
by different version by package update. When that happened,
rich-core-dumper reported wrong build-id for the binary.

Needs CONFIG_CHECKPOINT_RESTORE to be enabled in kernel.
Create buildids_missing section only if there is anything to put there
Collect libraries that were deleted into new a rich core section

For libraries in that section we weren't able to obtain build-id because
the binary was deleted or changed by package update and kernel doesn't
have CONFIG_CHECKPOINT_RESTORE enabled (i.e. dumper couldn't read
/proc/$pid/map_files).
Remove trailing whitespace from buildids_missing section
Merge pull request #23 from xhaakon/master

When available, use /proc/$pid/map_files to read build-ids