Change --ponyminthreads default to 0

Thanks to @slfritchie's hard work in ponylang#2926 and ponylang#2561 to find and
fix the bugs in the dynamic scheduler scaling logic, scheduler
thread 0 now also suspends and resumes correctly without issues
(in my testing).

This commit changes the default for `--ponyminthreads` from `1` to
`0`. The last time @slfritchie ran into issues was in March and
left a comment with the commands he used at:
ponylang#2561 (comment)

I ran the commands from @slfritchie's comment (with minor changes
to ensure `--ponyminthreads=0`) using Pony 0.26.0 for a few hours
without any issues.

src/libponyc/options/options.c

@@ -197,8 +197,7 @@ static void usage(void)
     "  --ponythreads    Use N scheduler threads. Defaults to the number of\n"
     "                   cores (not hyperthreads) available.\n"
     "  --ponyminthreads Minimum number of active scheduler threads allowed.\n"
-    "                   Defaults to 1.  Use of 0 is considered experimental\n"
-    "                   and may cause improper scheduler behavior.\n"
+    "                   Defaults to 0.\n"
     "  --ponysuspendthreshold\n"
     "                   Amount of idle time before a scheduler thread suspends\n"
     "                   itself to minimize resource consumption (max 1000 ms,\n"

src/libponyrt/sched/start.c

@@ -130,7 +130,7 @@ PONY_API int pony_init(int argc, char** argv)
   memset(&opt, 0, sizeof(options_t));
 
   // Defaults.
-  opt.min_threads = 1;
+  opt.min_threads = 0;
   opt.cd_detect_interval = 100;
   opt.gc_initial = 14;
   opt.gc_factor = 2.0f;