Bug #1625: Filename: Umlaute and special characters

Fix for https://tvheadend.org/issues/1625
I've add option for "Remove unsafe characters from filename but keep national characters" - it's less restrict than existing "Remove all unsafe characters from filename"

I've found than code for handling unsafe characters is duplicated in dvr_db.c (used for filename - safe char is "\_") and dvr_rec.c (used for directory name - safe char is "-"). 
I've refactor it to use code from dvr_cleanupfilename and always replace unsafe chars to "_" (only whitespaces are replaced as "-").