LLSDNotationFormatter (also LLSDNotationStreamer that uses it, plus
operator<<(std::ostream&, const LLSD&) that uses LLSDNotationStreamer) is most
useful for displaying LLSD to a human, e.g. for logging. Having the default
dump raw binary bytes into the log file is not only suboptimal, it can
truncate the output if one of those bytes is '\0'. (This is a problem with the
logging subsystem, but that's a story for another day.)

Use OPTIONS_PRETTY_BINARY wherever there is a default LLSDFormatter
::EFormatterOptions argument.

Also, allow setting LLSDFormatter subclass boolalpha(), realFormat() and
format(options) using optional constructor arguments. Naturally, each subclass
that supports this must accept and forward these constructor arguments to its
LLSDFormatter base class constructor.

Fix a couple bugs in LLSDNotationFormatter::format_impl() for an LLSD::Binary
value with OPTIONS_PRETTY_BINARY:
- The code unconditionally emitted a b(len) type prefix followed by either raw
  binary or hex, depending on the option flag. OPTIONS_PRETTY_BINARY caused it
  to emit "0x" before the hex representation of the data. This is wrong in
  that it can't be read back by either the C++ or the Python LLSD parser.
  Correct OPTIONS_PRETTY_BINARY formatting consists of b16"hex digits" rather
  than b(len)"raw bytes".
- Although the code did set hex mode, it didn't set either the field width or
  the fill character, so that a byte value less than 16 would emit a single
  digit rather than two.

Instead of having one LLSDFormatter::format() method with an optional options
argument, declare two overloads. The format() overload without options passes
the mOptions data member to the overload accepting options.

Refactor the LLSDFormatter family, hoisting the recursive format_impl() method
(accepting level) to a pure virtual method at LLSDFormatter base-class level.
Most subclasses therefore need not override either base-class format() method,
only format_impl(). In fact the short format() overload isn't even virtual.

Consistently use LLSDFormatter::EFormatterOptions enum as the options
parameter wherever such options are accepted.