LibC: Add posix_openpt(), grantpt() and unlockpt()

This makes getting a pseudoterminal pair a little bit more portable.
Note that grantpt() and unlockpt() are currently no-ops, since we've
already granted the pseudoterminal slave to the calling user.

We also accept O_CLOEXEC to posix_openpt(), unlike some systems. :^)

Libraries/LibC/stdlib.cpp

@@ -746,4 +746,26 @@ char* realpath(const char* pathname, char* buffer)
     errno = 0;
     return buffer;
 }
+
+int posix_openpt(int flags)
+{
+    if (flags & ~(O_RDWR | O_NOCTTY | O_CLOEXEC)) {
+        errno = EINVAL;
+        return -1;
+    }
+
+    return open("/dev/ptmx", flags);
+}
+
+int grantpt(int fd)
+{
+    (void)fd;
+    return 0;
+}
+
+int unlockpt(int fd)
+{
+    (void)fd;
+    return 0;
+}
 }