Refactoring the mmap interface. The controlling pragma is now "mmap_size"

instead of "mmap_limit".  Also change SQLITE_CONFIG_MMAP_LIMIT and
SQLITE_FCNTL_MMAP_LIMIT to SQLITE_CONFIG_MMAP_SIZE and
SQLITE_FCNTL_MMAP_SIZE, respecctively.  
The default mmap_size is now always 0, meaning that
memory mapped I/O is off by default.  There is a new compile-time option
SQLITE_MAX_MMAP_SIZE that determines a hard upper bound on the mmap_size.
Setting SQLITE_MAX_MMAP_SIZE to zero disables the memory-mapped I/O logic
and causes it to be omitted from the build.  An extra argument is added
to SQLITE_CONFIG_MMAP_SIZE that can optionally lower the SQLITE_MAX_MMAP_SIZE
at start-time. The SQLITE_MAX_MMAP_SIZE is zero for platforms where we 
know that it does not work, meaning that it cannot be turned on by mistake
on those platforms.

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