Closes #410

This feature requires `libpcre`.

The protocol documentation says that the difference between `find` and
`search` is that `search` is case insensitive, but that's only half
the truth: `search` also searches for sub strings instead of matching
the whole string.  This part is undocumented and unfortunate, but at
this point, we can't change it.

However leaking this surprising behavior to the new filter expressions
was a bad idea; the "==" operator should never match substrings.  For
people who need that, we should add a new operator.

Closes #397

Prepare to allow more complex expressions.

Required for "modified-since".

All DetachedSong instances should be converted to LightSong first,
which is a very cheap operation.

Closes #89

Will allow more complex fitler expression, such as negation (#89).

Not yet wired to the protocol.

Consume the ConstBuffer instead of iterating it.  This will allow
adding more complex parsers.

This enforces the "not nullptr" rule.

Error message sent to client was "basic_string::_M_construct null not
valid" due to passing nullptr to the std::string constructor.

Regression caused by commit 386688b8

Error message sent to client was "basic_string::_M_construct null not
valid" due to passing nullptr to the std::string constructor.

Regression caused by commit 386688b8

Fixes search within mount points, resulting in error "No such
directory".

Closes #190