syscalls: Add 'scandir*'.
* guix/build/syscalls.scm (%struct-dirent-header): New C struct.
(string->pointer/utf-8, pointer->string/utf-8): New procedures.
(opendir*, closedir*, readdir*, scandir*): New procedures.
* tests/syscalls.scm ("scandir*, ENOENT")
("scandir*, ASCII file names", "scandir*, UTF-8 file names")
("scandir*, properties): New tests.