with-lock-ex: Provide -t (timeout) option

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Signed-off-by: Matthew Vernon <mv3@sanger.ac.uk>
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v4: Initial capital and final full-stop on sentences
    Remove code that attempted to reduce post-lock timeout race
    Move timer clearing to outside the retry loop

v3: split off from conversion to getopt