lib/common.sh: Make it safe to include multiple times.

This is similar to common C #ifdef guards.

I was tempted to wrap the entire thing in the if/fi, rather than use
'return' to bail early.  However, that means it won't execute anything
until after it reaches 'fi'.  And if `shopt -s extglob` isn't executed
before parsing, then it will syntax-error on the extended globs.  One
solution would have been to move `shopt -s extglob` up above the
include-guard.  But the committed solution is all-around simpler.

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