Revert usage of /proc/PID/maps

Using maps breaks on backtraces with VDSO and the elfutils bug this may
have worked around has been fixed:
https://bugzilla.redhat.com/show_bug.cgi?id=1112610

Fixes #174.

Related elfutils RFE: https://bugzilla.redhat.com/show_bug.cgi?id=1129777

Signed-off-by: Martin Milata <mmilata@redhat.com>