Add some abstractions to simplify the `rbuf_but_last_n_lines()`
function, which implements the "take all but the last `n` lines"
functionality of the `head` program. This commit adds
- `RingBuffer`, a fixed-size ring buffer,
- `ZLines`, an iterator over zero-terminated "lines",
- `TakeAllBut`, an iterator over all but the last `n` elements of an
iterator.
These three together make the implementation of
`rbuf_but_last_n_lines()` concise.
Change the behavior of `head` to display an error for each problematic
file, instead of displaying an error message for the first problematic
file and terminating immediately at that point. This change now matches
the behavior of GNU `head`.
Before this commit, the first error caused the program to terminate
immediately:
$ head a b c
head: error: head: cannot open 'a' for reading: No such file or directory
After this commit:
$ head a b c
head: cannot open 'a' for reading: No such file or directory
head: cannot open 'b' for reading: No such file or directory
head: cannot open 'c' for reading: No such file or directory
Fix a bug in which `head` failed to print headings for `stdin` inputs
when reading from multiple files, and fix another bug in which `head`
failed to print a blank line between the contents of a file and the
heading for the next file when reading multiple files. The output now
matches that of GNU `head`.
