Capture whence information in all Serval logging API entry points.
Introduce the LogLevel enum type to avoid exposing internal C API
details in the Swift API.
Rename Swift API methods and types to use camelCase naming style, to
distinguish it from the C API, which uses under_score naming style.
The daemon API is built as a Swift module called ServalDNA.
The new CliContext class allows easy implementation of CLI output using
Swift code. The new CliContextFile subclass is the obvious first
implementation, equivalent to cli_stdio.c. The 'servaldswift'
executable now uses CliContextFile to print its CLI output to standard
output.
The new delegated log output support constructs each log line in a
buffer and prints it by calling the delegate's 'print' function at
end-of-line. The 'servaldswift' executable now provides a log output
delegate in Swift that simply prints to standard error, replacing
log_output_console.o, which is omitted from its link.
