Create AtomicWriter class, for atomically appending to files.
Reword AtomicWriter<T>::commit() to use mkstemp (safer).
Simplify "swx resume" command.

Abolish ability to pass arguments to "swx resume". This ability seemed a
bit confusing and in any case duplicated the functionality of "switch -r".

Also do away with "r" alias, which had the potential to be confused with
the "-r" option.
Merge branch 'master' into atomic-writer
Improve implementation of AtomicWriter, and simplify API for same.
Fixes to AtomicWriter; and incorporate AtomicWriter into TimeLog.
Fix formatting error in report printing.
Further improvements to AtomicWriter. Make use of it in Config.
Tidy-ups in AtomicWriter.
Merge branch 'atomic-writer'
Enable exceptions on streams, for simpler handling of unlikely errors.
Various fixes re. I/O.