gdbserver/tracepoint.cc: use snprintf in gdb_agent_socket_init

gdbserver/tracepoint.cc: use snprintf in gdb_agent_socket_init

If we modify tracepoint.cc to try to use a too long unix socket name,
for example by modifying SOCK_DIR to be:

    #define SOCK_DIR "/tmp/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut/salut"

... trying to start an application with libinproctrace.so loaded
crashes:

    $ LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libasan.so.6:./libinproctrace.so /bin/ls
    /home/smarchi/src/binutils-gdb/gdbserver/../gdbsupport/common-utils.cc:69: A problem internal to GDBserver in-process agent has been detected.
    xsnprintf: Assertion `ret < size' failed.

Looking at the rest of the socket initialization code, the intent seems
to be that if something goes wrong, we warn but let the program
execute.  So crashing on this failed assertions seems against the intent.

Commit 6cebaf6e1ae4 ("use xsnprintf instead of snprintf.") changed this
code to use xsnprintf instead of snprintf, introducing this assertion.
Before that, snprintf would return a value bigger that UNIX_PATH_MAX and
the "if" after would catch it and emit a warning, which is exactly what
we want.  That change was done because LynxOS didn't have snprintf.
Since LynxOS isn't supported anymore, we can simply revert to use
snprintf there.

With this patch, we get a warning (printed by the caller of
gdb_agent_socket_init), but the program keeps executing:

    $ LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libasan.so.6:./libinproctrace.so /bin/ls
    ipa: could not create sync socket
    ...

Change-Id: I78bca52d5dc3145335abeae45a42052701e3f5dd