Everything ties together in stream configurations, of which there can be many
now.

For the time being, every stream configuration but the "default" is ignored.

Every stream/server/encoder/decoder configuration now has the name "default"
by default, so that configuring names is only needed to resolve ambiguities.

Decoder configurations are now also constrained in this manner for consistency,
meaning they are no longer uniquely identified by a number of file extensions
but also a name.

* Do not shell-quote strings in the metadata token expansion function
  * This fixes https://github.com/xiph/ezstream/issues/6
* Expand all tokens at the same time in a way that ensures metadata
  is not additional input to expansion
* Simplify several functions and remove unused functionality

Fixed the copypasta mistake of using the PLACEHOLDER_ARTIST value in the PLACEHOLDER_ALBUM conditional.

This was a regression introduced by the cfg refactor

... and don't initialize global values twice while here

* Stop supporting systems without stat(2) while here
* Remove some additional configure checks for ubiquitous functions (>=C99)