Add property-db class to record & write structured data.

This adds a utility to record structured data in the form of a property database where the keys are a property path with string or number values. The data can be converted to a string, or written to a file, as JSON.

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Some toolsets are now warning that sprintf is insecure/deprecated. This
replaces all sprintf calls with equivalent b2 equivalents or new prtinf
style utilities. This is for all platforms. Which avoids having to debug
alternate implementations.

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The goal here is to avoid extra complexity of just building. If someone
needs more control they can use the src/engine/build scripts.

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