We rely on the include path to find many of these headers. It
especially doesn't make sense to use #include "foo.h" for headers
in the top-level include directory.
Avoids compiler warnings with OpenSSL 3.0. EVP_MD_CTX_new() is
only available for OpenSSL 1.1 and higher--we will fall back to
sudo's SHA2 code if necessary.
Previously we needed to include headers required by the various
sudo*h files. Now those files are more self-sufficient and we
should only include headers needed by code in the various .c files.
