lazutils: TrimFilename: optimized

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@43403 4005530d-fff6-0310-9dd1-cebe43e6787f
lazutils: TrimFilename: normalize pathdelims

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@43404 4005530d-fff6-0310-9dd1-cebe43e6787f