Move platform specific to sys/

Move unix/ and win/ to sys/, so that we can use android/ for android
headers without confusion.

app/src/sys/unix/command.c

@@ -0,0 +1,39 @@
+#include "../../command.h"
+
+#include <signal.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+pid_t cmd_execute(const char *path, const char *const argv[]) {
+    pid_t pid = fork();
+    if (pid == -1) {
+        perror("fork");
+        return -1;
+    }
+    if (pid == 0) {
+        execvp(path, (char *const *)argv);
+        perror("exec");
+        _exit(1);
+    }
+    return pid;
+}
+
+SDL_bool cmd_terminate(pid_t pid) {
+    return kill(pid, SIGTERM) != -1;
+}
+
+SDL_bool cmd_simple_wait(pid_t pid, int *exit_code) {
+    int status;
+    int code;
+    if (waitpid(pid, &status, 0) == -1 || !WIFEXITED(status)) {
+        // cannot wait, or exited unexpectedly, probably by a signal
+        code = -1;
+    } else {
+        code = WEXITSTATUS(status);
+    }
+    if (exit_code) {
+        *exit_code = code;
+    }
+    return !code;
+}

app/src/sys/win/command.c

@@ -0,0 +1,44 @@
+#include "../../command.h"
+
+#include <stdio.h>
+#include "../strutil.h"
+
+HANDLE cmd_execute(const char *path, const char *const argv[]) {
+    STARTUPINFO si;
+    PROCESS_INFORMATION pi;
+    memset(&si, 0, sizeof(si));
+    si.cb = sizeof(si);
+
+    // Windows command-line parsing is WTF:
+    // <http://daviddeley.com/autohotkey/parameters/parameters.htm#WINPASS>
+    // only make it work for this very specific program
+    // (don't handle escaping nor quotes)
+    char cmd[256];
+    size_t ret = xstrjoin(cmd, argv, ' ', sizeof(cmd));
+    if (ret >= sizeof(cmd)) {
+        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Command too long (%" PRIsizet " chars)", sizeof(cmd) - 1);
+        return NULL;
+    }
+
+    if (!CreateProcess(NULL, cmd, NULL, NULL, FALSE, 0, NULL, NULL, &si, &pi)) {
+        return NULL;
+    }
+
+    return pi.hProcess;
+}
+
+SDL_bool cmd_terminate(HANDLE handle) {
+    return CloseHandle(handle);
+}
+
+SDL_bool cmd_simple_wait(HANDLE handle, DWORD *exit_code) {
+    DWORD code;
+    if (WaitForSingleObject(handle, INFINITE) != WAIT_OBJECT_0 || !GetExitCodeProcess(handle, &code)) {
+        // cannot wait or retrieve the exit code
+        code = -1; // max value, it's unsigned
+    }
+    if (exit_code) {
+        *exit_code = code;
+    }
+    return !code;
+}