Big rename fest of MingW shared libraries

So far, MingW shared libraries were named like this

  libeay32.dll + libeay32.dll.a
  ssleay32.dll + ssleay32.dll.a

That naming scheme is antiquated, a reminicense of SSLeay.  We're
therefore changing the scheme to something that's more like the rest
of OpenSSL.

There are two factors to remember:

  - Windows libraries have no recorded SOvers, which means that the
    shared library version must be encoded in the name.  According to
    some, it's unwise to encode extra periods in a Windows file name,
    so we convert version number periods to underscores.
  - MingW has multilib ability.  However, DLLs need to reside with the
    binaries that use them, so to allow both 32-bit and 64-bit DLLs to
    reside in the same place, we add '-x64' in the name of the 64-bit
    ones.

The resulting name scheme (for SOver 1.1) is this:

  on x86:
  libcrypto-1_1.dll + libcrypto.dll.a
  libssl-1_1.dll + libssl.dll.a

  on x86_64:
  libcrypto-1_1-x64.dll + libcrypto.dll.a
  libssl-1_1-x64.dll + libssl.dll.a

An observation is that the import lib is the same for both
architectures.  Not to worry, though, as they will be installed in
PREFIX/lib/ for x86 and PREFIX/lib64/ for x86_64.

As a side effect, MingW got its own targets in Makefile.shared.
link_dso.mingw-shared and link_app.mingw-shared are aliases for the
corresponding cygwin-shared targets.  link_shlib.mingw-shared is,
however, a target separated from the cygwin one.

Reviewed-by: Andy Polyakov <appro@openssl.org>