scripts/clean-includes: added duplicate #include check

Enhance the clean-includes script to optionally check for duplicate #include
entries.

Script might output false positive entries as well. Such entries should
not be removed. So if it finds any duplicate entries script will
terminate with an exit status 1. Then each and every file should be
checked manually and corrected if necessary.

In order to enable the check use --check-dup-head option with
scripts/clean-includes.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Anand J <anand.indukala@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
