Fix stdout output to support specified driver, and add loop-until feature

[HunterZ]

Fixes #60 and is at least a possible fix for #14. Both of these changes help with using xmp-cli as a streaming broadcast decoder:

The -c and -o options now cache off the output driver as a "guess" instead of forcing things, and then only applies it to driver_id if the user didn't explicitly specify one via -d. The idea is to allow the user to output a specific audio format to stdout instead of just raw bytes.

A -U / --loop-until option now allows looping to occur until playtime has reached the specified number of seconds. The idea is to allow short loopable mods to play for a bit longer before moving on, so that they don't blow by too quickly in a playlist.

Specific code changes:

• common.h:
  • add loop_time to struct options
• main.c:
  • allow loop if loop_time defined and playtime has not exceeded it
• options.c:
  • add "loop-until" feature
  • change '-c' and '-o' logic to record output driver as a "guess" that only gets applied if no driver is explicitly specified

[sezero]

About fixing #60: We have to seek back to start of file to write a proper wav header: How is this really helping?

[HunterZ]

About fixing #60: We have to seek back to start of file to write a proper wav header: How is this really helping?

• sound_wav.c explicitly supports writing to stdout
• it works just fine with liquidsoap when I use it this way
• I get the same MD5 hash from either of the following with these changes:

  
  ./xmp -b16 -dwav -oben.wav -f44100 --nocmd -Z out.of.silence.2.mod
  ./xmp -b16 -c -dwav -f44100 --nocmd -Z out.of.silence.2.mod > ben2.wav

85efbdb2bfda69973e69ff0a62eb3b58 ben2.wav 85efbdb2bfda69973e69ff0a62eb3b58 ben.wav

Again, liquidsoap *needs* a WAV header on stdout, period, so my only other option is to do something silly like pipe through ffmpeg for no good reason. This fix works - I just DJ'd for a half hour on anonradio.net with liquidsoap invoking xmp-cli to render songs to WAV on stdout, which it then encoded to MP3 and passed to an icecast server.

Edit: I guess the redirect test is misleading because apparently `fseek()` works when redirecting stdout to a file but not when writing to a terminal or piping to another program? I guess maybe liquidsoap is just tolerant of that data being missing, or maybe redirects to a temporary file under the hood.

Edit 2: Looks like some software is tolerant (VLC, liquidsoap) and some isn't (Cubic Player), so this is only going to be situationally useful.

[sezero]

@AliceLR ?

[sezero]

OK, then: merging after @AliceLR's approval.