libCom: joinable threads (posix only)

Add epicsThreadJoin() and epicsThreadOpts::joinable.
For compatibility, default threads aren't joinable
by default.

Currently only POSIX can join.

modules/libcom/src/osi/epicsThread.cpp

@@ -38,6 +38,7 @@ epicsThreadId epicsShareAPI epicsThreadCreate (
     epicsThreadOpts opts;
     opts.priority = priority;
     opts.stackSize = stackSize;
+    opts.joinable = 0;
 
     return epicsThreadCreateOpt(name, funptr, parm, &opts);
 }