Add a String::format() and use that in place of ksprintf() in the Kernel.

You're never gonna be right 100% of the time when guessing how much buffer
space you need. This avoids having to make that type of decision in a bunch
of cases. :^)

Kernel/SlavePTY.cpp

@@ -19,9 +19,7 @@ SlavePTY::~SlavePTY()
 
 String SlavePTY::tty_name() const
 {
-    char buffer[32];
-    ksprintf(buffer, "/dev/pts/%u", m_index);
-    return buffer;
+    return String::format("/dev/pts/%u", m_index);
 }
 
 void SlavePTY::on_master_write(const byte* buffer, size_t size)