refactor: move the native devkit from 0:/usr to 0:/sdk (Montauk SDK)

0:/usr was a Unix transplant in an otherwise Montauk-native layout.
The SDK now lives at 0:/sdk/{bin,include,lib} (+ ldscripts under
0:/sdk/x86_64-montauk): Montauk-flavored, and a clean unit for a
future per-component installer tickbox. Native binutils is rebuilt
with --prefix=/sdk so ld's compiled-in search paths follow; future
ports configure with --prefix=/sdk.

Also add pathconf() with _PC_* names to the libc: now that realpath
exists, libiberty's lrealpath.c compiles its pathconf fallback path
unconditionally.

Shell command resolution and tab completion updated to 0:/sdk/bin.
Boot-verified in QEMU: setup, login, desktop on the new image with
zero usr/ remnants in the ramdisk.

Co-Authored-By: Claude Fable 5 <[email protected]>

programs/include/libc/unistd.h

@@ -38,6 +38,15 @@ int    execv(const char *path, char *const argv[]);
 int    execve(const char *path, char *const argv[], char *const envp[]);
 int    execvp(const char *file, char *const argv[]);
 int    pipe(int fds[2]);
+long   pathconf(const char *path, int name);
+
+/* pathconf() names (Linux numbering). */
+#define _PC_LINK_MAX  0
+#define _PC_MAX_CANON 1
+#define _PC_MAX_INPUT 2
+#define _PC_NAME_MAX  3
+#define _PC_PATH_MAX  4
+#define _PC_PIPE_BUF  5
 
 extern char **environ;