Kernel+LibC: Move name length constants to Kernel/API from limits.h

Reduce inclusion of limits.h as much as possible at the same time.

This does mean that kmalloc.h is now including Kernel/API/POSIX/limits.h
instead of LibC/limits.h, but the scope could be limited a lot more.
Basically every file in the kernel includes kmalloc.h, and needs the
limits.h include for PAGE_SIZE.

Kernel/Syscalls/mmap.cpp

@@ -20,7 +20,6 @@
 #include <Kernel/PerformanceEventBuffer.h>
 #include <Kernel/PerformanceManager.h>
 #include <Kernel/Process.h>
-#include <LibC/limits.h>
 #include <LibELF/Validation.h>
 
 namespace Kernel {