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/ELFLoader.cpp

@@ -205,15 +205,11 @@ char* ELFLoader::symbol_ptr(const char* name)
 bool ELFLoader::allocate_section(LinearAddress laddr, size_t size, size_t alignment, bool is_readable, bool is_writable)
 {
     ASSERT(alloc_section_hook);
-    char namebuf[16];
-    ksprintf(namebuf, "elf-alloc-%s%s", is_readable ? "r" : "", is_writable ? "w" : "");
-    return alloc_section_hook(laddr, size, alignment, is_readable, is_writable, namebuf);
+    return alloc_section_hook(laddr, size, alignment, is_readable, is_writable, String::format("elf-alloc-%s%s", is_readable ? "r" : "", is_writable ? "w" : ""));
 }
 
 bool ELFLoader::map_section(LinearAddress laddr, size_t size, size_t alignment, size_t offset_in_image, bool is_readable, bool is_writable)
 {
     ASSERT(alloc_section_hook);
-    char namebuf[16];
-    ksprintf(namebuf, "elf-map-%s%s", is_readable ? "r" : "", is_writable ? "w" : "");
-    return map_section_hook(laddr, size, alignment, offset_in_image, is_readable, is_writable, namebuf);
+    return map_section_hook(laddr, size, alignment, offset_in_image, is_readable, is_writable, String::format("elf-map-%s%s", is_readable ? "r" : "", is_writable ? "w" : ""));
 }

Kernel/FileDescriptor.cpp

@@ -292,16 +292,10 @@ String FileDescriptor::absolute_path()
     Stopwatch sw("absolute_path");
     if (is_tty())
         return tty()->tty_name();
-    if (is_fifo()) {
-        char buf[32];
-        ksprintf(buf, "fifo:%x", m_fifo.ptr());
-        return buf;
-    }
-    if (is_character_device()) {
-        char buf[128];
-        ksprintf(buf, "device:%u,%u (%s)", m_device->major(), m_device->minor(), m_device->class_name());
-        return buf;
-    }
+    if (is_fifo())
+        return String::format("fifo:%x", m_fifo.ptr());
+    if (is_character_device())
+        return String::format("device:%u,%u (%s)", m_device->major(), m_device->minor(), m_device->class_name());
     ASSERT(m_inode);
     return VFS::the().absolute_path(*m_inode);
 }

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)

Kernel/ProcFileSystem.cpp

@@ -153,9 +153,7 @@ ByteBuffer procfs$pid_cwd(Process& process)
 void ProcFS::add_process(Process& process)
 {
     InterruptDisabler disabler;
-    char buf[16];
-    ksprintf(buf, "%d", process.pid());
-    auto dir = add_file(create_directory(buf));
+    auto dir = add_file(create_directory(String::format("%d", process.pid())));
     m_pid2inode.set(process.pid(), dir.index());
     add_file(create_generated_file("vm", [&process] (SynthFSInode&) { return procfs$pid_vm(process); }), dir.index());
     add_file(create_generated_file("vmo", [&process] (SynthFSInode&) { return procfs$pid_vmo(process); }), dir.index());

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)

Kernel/VirtualConsole.cpp

@@ -508,9 +508,7 @@ void VirtualConsole::on_tty_write(const byte* data, size_t size)
 
 String VirtualConsole::tty_name() const
 {
-    char buf[16];
-    ksprintf(buf, "/dev/tty%u", m_index);
-    return String(buf);
+    return String::format("/dev/tty%u", m_index);
 }
 
 void VirtualConsole::set_vga_start_row(word row)