librtld_db: Handle shlibs with discontiguous mappings.

Some shared libraries specify mappings that leave a gap (actually a
MAP_GUARD mapping) in between the file mappings (libcrypto.so was
the one I found), and this would cause rd_loadobj_iter() to report
the mapping info incorrectly, leaving out rdl_path and misreporting
rdl_offset for file mappings after the gap. Fix rd_loadobj_iter()
to handle this situation.

Reviewed by: markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D32950