Downgrade fatal to warning in linux_async

Downgrade fatal to warning in linux_async

This commit downgrades a fatal error to a warning in linux_async.
linux_async is called from two different places in gdbserver:

 Via target_async from handle_accept_event.  The argument
   is always zero, so the warning will never be printed here.

 Via start_non_stop from handle_general_set.  This prints
   its own error message to stderr on failure, which will
   be preceded by the warning if it is emitted.

gdb/gdbserver/ChangeLog:

	* linux-low.c (linux_async): Replace fatal with warning.
	Tidy up and return.
	(linux_start_non_stop): Return -1 if linux_async failed.