CPP-35902: Fix SHELL variable when running under WSL with ttyfix

Due to https://github.com/microsoft/WSL/issues/10718 `SHELL` var is set to `ttyfix` breaking `gdb`.

We now:
1. Check if first argument (command to execute) is a valid shell and set `SHELL` accordingly
2. If not, check user default shell. Be it a valid shell, we use it
3. Unset `SHELL` otherwise

Review:
https://jetbrains.team/p/ij/reviews/118965/timeline

GitOrigin-RevId: 425b6886b4b48e29f653413c62326afa8033c694

native/WslTools/ttyfix.c

@@ -2,10 +2,41 @@
 #include <stdio.h>
 #include <unistd.h>
 #include <sys/ioctl.h>
+#include <stdlib.h>
+#include <string.h>
+#include <pwd.h>
+
+
+// Find appropriate shell
+// command_to_execute is a first command provided to ttyfix
+static char *detect_shell(char *command_to_execute) {
+    struct passwd *passwd_entry = getpwuid(geteuid());
+    char *user_shell = (passwd_entry != NULL) ? passwd_entry->pw_shell : NULL; //shell from /etc/passwd
+    int user_shell_ok = 0;
+    int command_ok = 0;
+
+    char *valid_shell;
+    while ((valid_shell = getusershell()) != NULL) { //List all valid shells from /etc/shells
+        if ((!user_shell_ok) && user_shell != NULL && (strcmp(user_shell, valid_shell) == 0)) {
+            user_shell_ok = 1; // User shell is ok, but we prefer to check command
+        }
+        if ((strcmp(valid_shell, command_to_execute) == 0)) { // First command is a valid shell, use it
+            command_ok = 1;
+            break; // Since command is the best choice, no need to check other shells
+        }
+    }
+    endusershell();
+    if (command_ok) {
+        return command_to_execute; // Command provided by user
+    } else if (user_shell_ok) {
+        return user_shell; // Use shell from /etc/passwd
+    }
+    return NULL;
+}
 
 // Workaround for https://github.com/microsoft/WSL/issues/10701
 // Provide command to execute along with args. Command will see 100x100 terminal
-int main(int argc, char *argv[], char *envp[]) {
+int main(int argc, char *argv[]) {
     if (argc < 2) {
         fprintf(stderr, "No command provided");
         return -1;
@@ -24,6 +55,16 @@ int main(int argc, char *argv[], char *envp[]) {
         close(fd);
     }
 
-    return execve(argv[1], argv + 1, envp); // Substitute self with provided command
+    // WSL sets SHELL to ttyfix: https://github.com/microsoft/WSL/issues/10718
+    char *shell_env = getenv("SHELL");
+    if (shell_env != NULL && (strcmp(argv[0], shell_env) == 0)) { // If SHELL == ttyfix
+        unsetenv("SHELL");
+        // Do our best to guess shell, or unset it otherwise
+        char *correct_shell = detect_shell(argv[1]);
+        if (correct_shell != NULL) {
+            setenv("SHELL", correct_shell, 1);
+        }
+    }
+    return execv(argv[1], argv + 1); // Substitute self with provided command
 }