OpenSystemsDevelopment / qpdf

Lazily load MD5 and RC4 once in the life of the program. Only load the
legacy provider if RC4 is actually being used.

Prior to the cmake conversion, several private classes had methods
that were exported into the shared library so they could be tested
with libtests. With cmake, we build libtests using an object library,
so this is no longer necessary. The methods that are disappearing from
the ABI were never exposed through public headers, so no code should
be using them. Removal had to wait until the window for ABI-breaking
changes was open.

Run this:

for i in  **/*.cc **/*.c **/*.h **/*.hh; do
  clang-format < $i >| $i.new && mv $i.new $i
done

OPENSSL_IS_BORINGSSL is not actually set by configure, so it will be
undefined until a BoringSSL header is included.  Hence the #ifdef logic
in QPDFCrypto_openssl.h would usually never apply.

This still worked because evp.h transitively included BoringSSL's
cipher.h and digest.h, but the latter are the correct (documented)
headers.

By re-ordering the includes, we can ensure the macro is defined when we
use it.

Also: fix case in the header guards.

Fixes qpdf/qpdf#417