Misc improvements

As discussed in partman-basicfilesystems!9 (merged) "Disable automatic use and forceful format of existing swaps":

• Strip the format{ } specifier from reused partitions
• Check the minimum size of reused partitions
• Support multiple space-separated disk labels in $iflabel{ }
• Allow EFI partitions not only on gpt but also on msdos disk label in built-in recipes