Move e_os.h to include/internal

Move e_os.h to include/internal

Including e_os.h with a path from a header file doesn't work well on
certain exotic platform.  It simply fails to build.

Since we don't seem to be able to stop ourselves, the better move is
to move e_os.h to an include directory that's part of the inclusion
path given to the compiler.

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17641)