os_linux.cpp: Add '-d by-id' option to device scanning.

If specified, scan '/dev/disk/by-id/*' for symlinks to '/dev/sdX'
and remove duplicates.

git-svn-id: https://svn.code.sf.net/p/smartmontools/code/trunk@4847 4ea69e1a-61f1-4043-bf83-b5c94c648137