[gdb/testsuite] Handle unreachable network in server-connect.exp

[gdb/testsuite] Handle unreachable network in server-connect.exp

When running gdb.server/server-connect.exp I run into:
...
FAIL: gdb.server/server-connect.exp: tcp6: connect to gdbserver using tcp6:::1
FAIL: gdb.server/server-connect.exp: tcp6-with-brackets: connect to gdbserver \
  using tcp6:[::1]
FAIL: gdb.server/server-connect.exp: udp6: connect to gdbserver using udp6:::1
FAIL: gdb.server/server-connect.exp: udp6-with-brackets: connect to gdbserver \
  using udp6:[::1]
...

The FAIL is caused by the fact that the ipv6 loopback address is not available:
...
PASS: gdb.server/server-connect.exp: tcp6: start gdbserver
target remote tcp6:::1:2347^M
A program is being debugged already.  Kill it? (y or n) y^M
tcp6:::1:2347: Network is unreachable.^M
(gdb) FAIL: gdb.server/server-connect.exp: tcp6: connect to gdbserver using tcp6:::1
...
This should be marked UNSUPPORTED rather than FAIL.

Furthermore, the test-case takes about 4 minutes, because the 'Network is
unreachable' response is not explicitly handled in gdb_target_cmd, so instead
it runs into the timeout case.

Fix this by handling the 'Network is unreachable' response as UNSUPPORTED.
This reduces testing time from 4 minutes to about 2 seconds.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-09-19  Tom de Vries  <tdevries@suse.de>

	* lib/gdbserver-support.exp (gdb_target_cmd_ext): Return 2 (meaning
	UNSUPPORTED) for 'Network is unreachable' message.  Factor out of ...
	(gdb_target_cmd): ... here.
	* gdb.server/server-connect.exp: Use gdb_target_cmd_ext, handle return
	value 2.