ENH: support search options on more dictionary methods

- can now specify literal matches for sub-dictionary methods:

    isDict(key, keyType::REGEX)
    optionalSubDict(key, keyType::REGEX)
    subDict(key, keyType::REGEX)
    subOrEmptyDict(key, keyType::REGEX, mandatory)

There is no change in behaviour of the methods, just the search option
is now exposed as an optional parameter.

NOTE: minor breaking change for subOrEmptyDict()

  old: subOrEmptyDict(key, bool=false)
  new: subOrEmptyDict(key, keyType::option=keyType::REGEX, bool=false)

  This affects code that previously explicitly set the bool parameter.
  Within OpenFOAM itself, this only affected a single file:

      KinematicCloud.C