Sync src with amalgamated versions

Now src contains exactly what's embedded in mongoose.c and .h, nothing more.

Added `tools/amalgam.sh` to aamlgamate both files at once.

There are no functional changes to mongoose.c, .h in this PR, only slight filename changes.

added MWS white paper

Download “9 Things NOT to do when embedding a web server” white paper.

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No mongoose.{c,h} diffs

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 * Actually drop the connection when no response to ping arrives within the next interval.
 * Avoid sending immediate ping when wall time is adjusted, it's usually spurious.

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If file ends with .gz and has known "secondary extnesion", i.e. test.html.gz,
its content type is determined by the secondary extension and content-encoding is set to gzip.

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Also free up some RAM by not allocating ca_cert when `MBEDTLS_X509_CA_CHAIN_ON_DISK` is enabled.

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CL: Add mg_strcasecmp()

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