Added X-Forwarded-{For,Proto} support.

Initially from https://github.com/mattsta/stud/commit/713edb27aa90b21b6ddd8ac89ad98ff68e8d28da, updated to apply to latest bumptech/stud master. Uses joyent's http-parser, specifically commit 1786fdae36d3d40d59463dacab1cfb4165cf9f1d.

The changes are *completely* isolated, to enable it compile stud using "USE_HTTP_HEADERS=1 make".

Original commit message follows:
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This is an early implementation of adding XFF support.

Notes:
  - You can't just append an XFF. You must detect any existing
    XFF headers and remove them.  We can't allow people to spoof
    originating IPs to our backends.
  - The changes are *completely* isolated.
  - When you run `make` you get two git downloads and three binaries:
    - stud -- normal stud.  no HTTP support is even compiled.
    - stud-http -- stud + http + x-forward-{for,proto} injection
    - stud-http-jem -- stud-http with statically linked jemalloc
  - There isn't a command line option to enable XFF (yet).  You get
    XFF forced on you when you run stud-http or stud-http-jem.

At this point, no performance tests have been done against
stud vs. stud-http vs. stud-http-jem.  This is an initial "get it
out there" commit.

The PROTO_HTTP changes introduce new malloc calls (hence adding
jemalloc), but all allocations are free'd as verified by
valgrind x86_64.  No reported leaks exist.

As an early implementation, things to be cleaned up include:
  - More stable error handling
  - Better code organization
  - More inline documentation
  - Performance tests and tradeoffs with prepending IP octets versus XFF
  - \n vs. \r vs. \r\n for re-building the HTTP headers

For parsing the HTTP headers, I use ry's portable and performant
http-parser.  To (hopefully) improve memory usage, I pull in jemalloc.

Refactoring and performance benchmarks are forthcoming.

Enjoy.
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Added SNI with wildcard certs support.

Just applied https://github.com/bumptech/stud/pull/126.

Original pull request message:
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Added a function to do basic wildcard matching when deciding which cert to use under SNI.

Also added some comments to the default config clarifying how to list multiple cert files.
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