Do not use 64-bit file IO calls in mmfile

These functions are not available on iOS or Android.

If libbitcoin is being compiled for a 64-bit platform, the standard
functions already work fine (size_t and off_t are already 64-bits).

If libbitcoin is being compiled for a 32-bit platform, files larger than
64GiB won't fit in the address space anyhow, so there is no point in being
64-bit aware here.
Use the right constant for madvise

The POSIX_MADV_SEQUENTIAL constant is for posix_madvise, not madvise.

Also, this constant doesn't exist on Android.
Re-add a function used by the libwallet URI parser

The libwallet URI parser needs to be identify characters as being base58 or not.
To avoid duplication of code, it makes sense to expose this from
libbitcoin's base58 utility file.
Merge pull request #38 from swansontec/master

Fix libbitcoin build errors for iOS, OS X, and Android