[gdb/server] Emit warning for SIGINT failure

[gdb/server] Emit warning for SIGINT failure

Consider the executable from test-case gdb.base/interrupt-daemon.exp.

When starting it using gdbserver:
...
$ ./build/gdbserver/gdbserver localhost:2345 \
  ./outputs/gdb.base/interrupt-daemon/interrupt-daemon
...
and connecting to it using gdb:
...
$ gdb -q -ex "target remote localhost:2345" \
    -ex "set follow-fork-mode child" \
    -ex "break daemon_main" -ex cont
...
we are setup to do the same as in the test-case: interrupt a running inferior
using ^C.

So let's try:
...
(gdb) continue
Continuing.
^C
...
After pressing ^C, nothing happens.  This a known problem, filed as
PR remote/18772.

The problem is that in linux_process_target::request_interrupt, a kill is used
to send a SIGINT, but it fails.  And it fails silently.

Make the failure verbose by adding a warning, such that the gdbserver output
becomes more helpful:
...
Process interrupt-daemon created; pid = 15068
Listening on port 2345
Remote debugging from host ::1, port 35148
Detaching from process 15068
Detaching from process 15085
gdbserver: Sending SIGINT to process group of pid 15068 failed: \
  No such process
...

Note that the failure can easily be reproduced using the test-case and target
board native-gdbserver:
...
(gdb) continue^M
Continuing.^M
PASS: gdb.base/interrupt-daemon.exp: fg: continue
^CFAIL: gdb.base/interrupt-daemon.exp: fg: ctrl-c stops process (timeout)
...
as reported in PR server/23382.

Tested on x86_64-linux.
Approved-By: Simon Marchi <simon.marchi@efficios.com>