AP_HAL_Linux: force Thread stack to have minimum size

Check that Thread stack size is the minimum allowed

Also add 256k to HAL-independent request stack size

libraries/AP_HAL_Linux/Scheduler.cpp

@@ -359,7 +359,8 @@ bool Scheduler::thread_create(AP_HAL::MemberProc proc, const char *name, uint32_
         }
     }
 
-    thread->set_stack_size(stack_size);
+    // Add 256k to HAL-independent requested stack size
+    thread->set_stack_size(256 * 1024 + stack_size);
 
     /*
      * We should probably store the thread handlers and join() when exiting,

libraries/AP_HAL_Linux/Thread.cpp

@@ -17,6 +17,7 @@
 #include "Thread.h"
 
 #include <alloca.h>
+#include <limits.h>
 #include <sys/types.h>
 #include <stdio.h>
 #include <unistd.h>
@@ -243,7 +244,7 @@ bool Thread::set_stack_size(size_t stack_size)
         return false;
     }
 
-    _stack_size = stack_size;
+    _stack_size = MAX(stack_size, (size_t) PTHREAD_STACK_MIN);
 
     return true;
 }