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/MasterPTY.cpp

@@ -14,9 +14,7 @@ MasterPTY::~MasterPTY()
 
 String MasterPTY::pts_name() const
 {
-    char buffer[32];
-    ksprintf(buffer, "/dev/pts/%u", m_index);
-    return buffer;
+    return String::format("/dev/pts/%u", m_index);
 }
 
 ssize_t MasterPTY::read(Process&, byte* buffer, size_t size)