Configurations/README: reword bn_ops description.

                           to have the different variants in different

                           directories.

        bn_ops          => Building options (was just bignum options

                           in the earlier history of this option,

                           hence the name).  This a string of words

                           that describe properties on the designated

                           target platform, such as the type of

                           integers used to build up the bitnum,

                           different ways to implement certain ciphers

                           and so on.  To fully comprehend the

        bn_ops          => Building options (was just bignum options in

                           the earlier history of this option, hence the

                           name). This is a string of words that describe

                           algorithms' implementation parameters that

                           are optimal for the designated target platform,

                           such as the type of integers used to build up

                           the bignum, different ways to implement certain

                           ciphers and so on. To fully comprehend the

                           meaning, the best is to read the affected

                           source.

                           The valid words are:

                           BN_LLONG     use 'unsigned long long' in

                                        some bignum calculations.

                                        This has no value when

                                        SIXTY_FOUR_BIT or

                                        SIXTY_FOUR_BIT_LONG is given.

                           RC4_CHAR     makes the basic RC4 unit of

                                        calculation an unsigned char.

                           SIXTY_FOUR_BIT       processor registers

                                                are 64 bits, long is

                                                32 bits, long long is

                                                64 bits.

                           SIXTY_FOUR_BIT_LONG  processor registers

                                                are 64 bits, long is

                                                64 bits.

                           THIRTY_TWO_BIT       processor registers

                                                are 32 bits.

                           THIRTY_TWO_BIT       bignum limbs are 32 bits,

                                                this is default if no

                                                option is specified, it

                                                works on any supported

                                                system [unless "wider"

                                                limb size is implied in

                                                assembly code];

                           BN_LLONG             bignum limbs are 32 bits,

                                                but 64-bit 'unsigned long

                                                long' is used internally

                                                in calculations;

                           SIXTY_FOUR_BIT_LONG  bignum limbs are 64 bits

                                                and sizeof(long) is 8;

                           SIXTY_FOUR_BIT       bignums limbs are 64 bits,

                                                but execution environment

                                                is ILP32;

                           RC4_CHAR             RC4 key schedule is made

                                                up of 'unsigned char's;

                           RC4_INT              RC4 key schedule is made

                                                up of 'unsigned int's;

                           EXPORT_VAR_AS_FN     for shared libraries,

                                                export vars as

                                                accessor functions.