More robust tempdir management

[iainrb]

• File::Temp->newdir() ensures temporary directory is deleted as soon as it goes out of scope, instead of persisting until script exit. Important as archiving can create large numbers of temporary directories.
• Use the system tempdir (typically /tmp) to avoid cluttering up the working directory
• The sigtrap pragma ensures object destructors are called (thereby removing temp directories), even after Control-C or similar.