Handle "show remote memory-write-packet-size" when not connected

Handle "show remote memory-write-packet-size" when not connected

Currently "show remote memory-write-packet-size" says that the packet
size is limited to whatever is stored in the remote_state global, even
if not connected to a target.

When we get to support multiple instances of remote targets, there
won't be a remote_state global anymore, so that must be replaced by
something else.

Since it doesn't make sense to print the limit of the packet size of a
non-existing connection, this patch makes us say that the limit will
be further reduced when we connect.

The text is taken from the command's online help, which says:

 "The actual limit is further reduced dependent on the target."

Note that a value of "0" is special, as per "help set remote
memory-write-packet-size":

 ~~~
 Specify the number of bytes in a packet or 0 (zero) for the
 default packet size.
 ~~~

I've tweaked "show remote memory-write-packet-size" to include
"(default)" in the output in that case, like this:

 (gdb) show remote memory-write-packet-size
 The memory-write-packet-size is 0 (default). The actual limit will be further reduced dependent on the target.

While working on this, I noticed that an explicit "set remote
write-packet-size 0" does not makes GDB go back to the exact same
state as the default state when GDB starts up:

 (gdb) show remote memory-write-packet-size
 The memory-write-packet-size is 0. [...]
                                 ^^

 (gdb) set remote memory-write-packet-size 0
 (gdb) show remote memory-write-packet-size
 The memory-write-packet-size is 16384. [...]
                                 ^^^^^

The "16384" number comes from DEFAULT_MAX_MEMORY_PACKET_SIZE.

This happens because git commit a5c0808e221c ("gdb: remove packet size
limit") at
<https://sourceware.org/ml/gdb-patches/2015-08/msg00743.html>, added
this:

  /* So that the query shows the correct value.  */
  if (size <= 0)
    size = DEFAULT_MAX_MEMORY_PACKET_SIZE;

to set_memory_packet_size, but despite what the comment suggests, that
also has the side-effect of recording DEFAULT_MAX_MEMORY_PACKET_SIZE
in config->size.

Finally, DEFAULT_MAX_MEMORY_PACKET_SIZE only makes sense for "set
remote memory-write-packet-size fixed", so I've renamed it
accordingly, to make it a little bit clearer.

gdb/ChangeLog:
2018-05-22  Pedro Alves  <palves@redhat.com>

	* remote.c (DEFAULT_MAX_MEMORY_PACKET_SIZE): Rename to ...
	(DEFAULT_MAX_MEMORY_PACKET_SIZE_FIXED): ... this.
	(get_fixed_memory_packet_size): New.
	(get_memory_packet_size): Use it.
	(set_memory_packet_size): Don't override the config size with
	DEFAULT_MAX_MEMORY_PACKET_SIZE.
	(show_memory_packet_size): Use get_fixed_memory_packet_size.
	Don't refer to get_memory_packet_size if not connected to a remote
	target.  Show "(default)" if configured size is 0.

gdb/testsuite/ChangeLog:
2018-05-22  Pedro Alves  <palves@redhat.com>

	* gdb.base/remote.exp: Adjust expected output of "show remote
	memory-write-packet-size".  Add tests for "set remote
	memory-write-packet-size 0" and "set remote
	memory-write-packet-size fixed/limit".