Do not cast away const in agent_run_command

While investigating something else, I noticed some weird code in
agent_run_command (use of memcpy rather than strcpy).  Then I noticed
that 'cmd' is used as both an in and out parameter, despite being
const.

Casting away const like this is bad.  This patch removes the const and
fixes the memcpy.  I also added a static assert to assure myself that
the code in gdbserver is correct -- gdbserver is passing its own
buffer directly to agent_run_command.

Reviewed-By: Andrew Burgess <aburgess@redhat.com>

gdbserver/server.cc

@@ -51,6 +51,11 @@
 #include "gdbsupport/scoped_restore.h"
 #include "gdbsupport/search.h"
 
+/* PBUFSIZ must also be at least as big as IPA_CMD_BUF_SIZE, because
+   the client state data is passed directly to some agent
+   functions.  */
+gdb_static_assert (PBUFSIZ >= IPA_CMD_BUF_SIZE);
+
 #define require_running_or_return(BUF)		\
   if (!target_running ())			\
     {						\