head: port to Darwin and use simpler seeks

head: port to Darwin and use simpler seeks

This removes an unportable assumption that if lseek succeeds, the
file is capable of seeking.  See: http://bugs.gnu.org/17145
* src/head.c (elseek): New function, for consistency in reporting
lseek failures.
(elide_tail_bytes_file, elide_tail_lines_seekable)
(elide_tail_lines_file, head_lines, head): Use it.
(elide_tail_bytes_file, elide_tail_lines_file):
New args CURRENT_POS and SIZE.  All uses changed.  Don't bother
invoking lseek, since we know the file's pos and size now.
(elide_tail_bytes_file): Change a local from uintmax_t to off_t,
since it fits.
(head): Use lseek only on regular files, since its behavior on
unseekable devices is implementation-defined.
* NEWS: Document this.