Switch to MAJOR.MINOR.PATCH versioning and version 3.0.0-dev

We're strictly use version numbers of the form MAJOR.MINOR.PATCH.
Letter releases are things of days past.

The most central change is that we now express the version number with
three macros, one for each part of the version number:

    OPENSSL_VERSION_MAJOR
    OPENSSL_VERSION_MINOR
    OPENSSL_VERSION_PATCH

We also provide two additional macros to express pre-release and build
metadata information (also specified in semantic versioning):

    OPENSSL_VERSION_PRE_RELEASE
    OPENSSL_VERSION_BUILD_METADATA

To get the library's idea of all those values, we introduce the
following functions:

    unsigned int OPENSSL_version_major(void);
    unsigned int OPENSSL_version_minor(void);
    unsigned int OPENSSL_version_patch(void);
    const char *OPENSSL_version_pre_release(void);
    const char *OPENSSL_version_build_metadata(void);

Additionally, for shared library versioning (which is out of scope in
semantic versioning, but that we still need...