Simplify the handling of shared library version numbers

$(SHLIB_MAJOR).$(SHLIB_MINOR) is really a synonym for
$(SHLIB_VERSION_NUMBER), and is therefore an added complexity,
so better to use $(SHLIB_VERSION_NUMBER) directly.  SHLIB_MAJOR and
SHLIB_MINOR are now unused, but are kept around purely as information
in case someone relies on their existence.

At the same time, add support for custom shared library extensions
with the three new Makefile variables SHLIB_EXT, SHLIB_EXT_SIMPLE and
SHLIB_EXT_IMPORT.  By default, they hold the variants of shared
library extensions we support.  On mingw and cygwin, SHLIB_EXT_IMPORT
is defined; on all other Unix platforms, it's empty.

An example to get shared libraries with a slightly different SOVER name:

    $ make SHLIB_EXT='.$(SHLIB_VERSION_NUMBER).so'

Fixes #3902

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3964)