snap: add deprecation noticed to "snap run --gdb"

Now that we promoted "snap run --gdbserver" the "snap run --gdb"
option should be deprecated. We plan to not improve it and it
needs to run as root, the program and gdb share a pty and it's
overall not great. So let's mark it deprecated and point users
to `snap run --gdbserver` instead.

cmd/snap-gdb-shim/snap-gdb-shim.c

@@ -34,6 +34,8 @@ int main(int argc, char **argv) {
     }
     // signal gdb to stop here
     printf("\n\n");
+    printf("DEPRECATED: Please consider using --gdbserver instead.\n");
+    printf("\n");
     printf("Welcome to `snap run --gdb`.\n");
     printf("You are right before your application is execed():\n");
     printf("- set any options you may need\n");

cmd/snap/cmd_run.go

@@ -75,8 +75,9 @@ type cmdRun struct {
 	// This options is both a selector (use or don't use strace) and it
 	// can also carry extra options for strace. This is why there is
 	// "default" and "optional-value" to distinguish this.
-	Strace                string `long:"strace" optional:"true" optional-value:"with-strace" default:"no-strace" default-mask:"-"`
-	Gdb                   bool   `long:"gdb"`
+	Strace string `long:"strace" optional:"true" optional-value:"with-strace" default:"no-strace" default-mask:"-"`
+	// deprecated in favor of Gdbserver
+	Gdb                   bool   `long:"gdb" hidden:"yes"`
 	Gdbserver             string `long:"gdbserver" default:"no-gdbserver" optional-value:":0" optional:"true"`
 	ExperimentalGdbserver string `long:"experimental-gdbserver" default:"no-gdbserver" optional-value:":0" optional:"true" hidden:"yes"`
 	TraceExec             bool   `long:"trace-exec"`
@@ -108,7 +109,7 @@ and environment.
 			// TRANSLATORS: This should not start with a lowercase letter.
 			"strace": i18n.G("Run the command under strace (useful for debugging). Extra strace options can be specified as well here. Pass --raw to strace early snap helpers."),
 			// TRANSLATORS: This should not start with a lowercase letter.
-			"gdb": i18n.G("Run the command with gdb"),
+			"gdb": i18n.G("Run the command with gdb (deprecated, use --gdbserver instead)"),
 			// TRANSLATORS: This should not start with a lowercase letter.
 			"gdbserver": i18n.G("Run the command with gdbserver"),
 			// TRANSLATORS: This should not start with a lowercase letter.