client API: remove ytdl=no default

With the recent changes to the script it does not incur a startup delay
by default due to starting youtube-dl and waiting for it. This was the
main reason for making libmpv have a different default.

Starting sub processes from a library can still be a bit fishy, but I
think it's ok. Still mention it in the libmpv header. There were already
other cases where libmpv would start its own processes, such as the X11
backend calling xdg-screensaver. (The reason why this is fishy is
because UNIX process management sucks: SIGCHLD and the wait() syscall
make sub processes non-transparent and could potentially introduce
conflicts with code trying to use them.)

DOCS/interface-changes.rst

@@ -24,6 +24,7 @@ Interface changes
       --ad-spdif-dtshd, --softvol options
     - fix --external-files: strictly never select any tracks from them, unless
       explicitly selected (this may or may not be expected)
+    - --ytdl is now always enabled, even for libmpv
  --- mpv 0.28.0 ---
     - rename --hwdec=mediacodec option to mediacodec-copy, to reflect
       conventions followed by other hardware video decoding APIs

DOCS/man/options.rst

@@ -547,8 +547,7 @@ Program Behavior
     Enable the youtube-dl hook-script. It will look at the input URL, and will
     play the video located on the website. This works with many streaming sites,
     not just the one that the script is named after. This requires a recent
-    version of youtube-dl to be installed on the system. (Enabled by default,
-    except when the client API / libmpv is used.)
+    version of youtube-dl to be installed on the system. (Enabled by default.)
 
     If the script can't do anything with an URL, it will do nothing.
 

etc/builtin.conf

@@ -17,7 +17,6 @@ idle=yes
 terminal=no
 input-terminal=no
 osc=no
-ytdl=no
 input-default-bindings=no
 input-vo-keyboard=no
 # OSX/Cocoa global input hooks

libmpv/client.h

@@ -134,6 +134,8 @@ extern "C" {
  * - The FPU precision must be set at least to double precision.
  * - On Windows, mpv will call timeBeginPeriod(1).
  * - On memory exhaustion, mpv will kill the process.
+ * - In certain cases, mpv may start sub processes (such as with the ytdl
+ *   wrapper script).
  *
  * Encoding of filenames
  * ---------------------