gdb: Add per-remote target variables for memory read and write config

This patch adds per-remote target variables for the configuration of
memory read- and write packet size.  It is a further change to commit
"gdb: Make global feature array a per-remote target array" to apply the
fixme notes described in commit 5b6d1e4f "Multi-target support".

The former global variables for that configuration are still available
to allow the command line configuration for all future remote
connections.  Similar to the command line configuration of the per-
remote target feature array, the commands

- set remotewritesize (deprecated)
- set remote memory-read-packet-size
- set remote memory-write-packet-size

will configure the current target (if available).  If no target is
available, the default configuration for future remote connections is
adapted.  The show command will display the current remote target's
packet size configuration.  If no remote target is selected, the default
configuration for future connections will be shown.

It is required to adapt the test gdb.base/remote.exp which is failing
for --target_board=native-extended-gdbserver.  With that board GDB
connects to gdbserver at gdb start time.  Due to this patch two loggings
"The target may not be able to.." are shown if the command 'set remote
memory-write-packet-size fixed' is executed while a target is connected
for the current inferior.  To fix this, the clean_restart command is
moved to a later time point of the test.  It is sufficient to be
connected to the server when "runto_main" is executed.  Now the
connection time is similar to a testrun with
--target_board=native-gdbserver.

To allow the user to distinguish between the packet-size configuration
for future remote connections and for the currently selected target, the
commands' loggings are adapted.