Implement read_dir_entry () VFS op, CE add ls command

libsystem/system.c

@@ -64,3 +64,7 @@ int mail_receive (void* mesg, size_t mesg_size) {
 int get_procgroup (int pid) { return (int)do_syscall (SYS_GET_PROCGROUP, pid); }
 
 int get_exec_pid (void) { return (int)do_syscall (SYS_GET_EXEC_PID, 0); }
+
+int read_dir_entry (const char* path, struct dir_entry* entry, size_t entry_num) {
+  return (int)do_syscall (SYS_READ_DIR_ENTRY, path, entry, entry_num);
+}

libsystem/system.h

@@ -2,6 +2,7 @@
 #define _LIBMSL_M_SYSTEM_H
 
 #include <desc.h>
+#include <dir_entry.h>
 #include <stddef.h>
 #include <stdint.h>
 
@@ -77,4 +78,7 @@ int get_procgroup (int pid);
 /* get PID of process, which exec'ed the current process */
 int get_exec_pid (void);
 
+/* Read directory entry */
+int read_dir_entry (const char* path, struct dir_entry* entry, size_t entry_num);
+
 #endif // _LIBMSL_M_SYSTEM_H