Add the SQLITE_CONFIG_MMAP_LIMIT configuration option for overriding the

Add the SQLITE_CONFIG_MMAP_LIMIT configuration option for overriding the
SQLITE_DEFAULT_MMAP_LIMIT compile-time setting.  Enhance "PRAGMA mmap_limit"
so that without a specific database name, it sets the limit on all database
files and changes the default for any future databases that might be added
using ATTACH.

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