Implement allowDiskUse() shell helper for find command in older branches

Only 4.4 clusters will support the allowDiskUse option for external sort with the find command. However, in order to allow old shells to be forwards compatible with a 4.4 cluster, we may wish to backport shell support for the allowDiskUse() helper. This helper was implemented in a 4.3 development version under SERVER-42077, which also made changes to mongod. This ticket tracks the work to separate the shell work from the remainder of the commit for SERVER-42077, and apply these changes to the 4.2 branch.