This change adds missing features:

* %[ for pattern matching with a scanset.
* %m for allocation of strings.
* %p for pointers.
* Field width for integers.

The following features remain unimplemented, like with printf(3):

* Floating point support.
* Wide character support.
* %n$ positional parameters.

The code has been completely refactored to be much more maintainable.

The implemented features should now be standards compliant. A large number
of edge cases have been fixed.

The vscanf_callback(3) function has been renamed to vcbscanf(3) and a new
cbscanf(3) function has been added.

The default linker script places .text.unlikely prior to .text.

The change 9d29e96c "Fix open(2) allowing
opening directories invalidly and check O_TRUNC errors." broke the chmod(2),
chown(2), and utimens(2) system calls on directories, because they can no
longer be opened for writing.

This changes fixes the regression by opening such paths for reading. There
is currently no filesystem permission checks for those system calls. However,
those system calls should check the permissions at the time of the operation
rather than relying on the file having been opened for writing previously.

The language in POSIX mentioning overriding blocking or ignoring SIGABRT
refers to the inevitability of exiting by SIGABRT if SIGABRT isn't caught or
if the handler does return.

This implementation of abort(3) implements the standard by raising SIGABRT,
allowing the signal to be caught; and if the signal is blocked or ignored or
the handler returns, then exit_thread(2) forcefully exits the process as if
by SIGABRT.

This fixes a regression in 62bd9bf9.

Among other things, redirecting to a directory will now display an error
as it should.

Also fix a bug when opening /dev/pts: O_WRITE on a directory is a POSIX
violation.

The child processes of pid 1 were being reparented to pid 1, causing an
infinite loop. This change fixes the problem by adding a hook that runs in
the last thread about to exit in a process. When pid 1 exits, the hook will
prevent more processes and threads from being created, and then broadcast
kill all processes and threads. The hook is not run in LastPrayer(), as that
function runs in a worker thread and it can't block waiting for another
thread to run LastPrayer() in the same thread.

POSIX allows <fcntl.h> to include <sys/stat.h>, but doesn't require it.
There's little reason to do that, since they are separate headers, and
<fcntl.h> just needs the mode_t constants. Fix the code accidentally
relying on <fcntl.h> including <sys/stat.h>. The mode_t constants are now
provided in their own kernel header <sortix/mode.h>.

Additionally fix <sys/stat.h> pulling in all of <sys/types.h>, which is not
allowed by POSIX, which only requires a few types to be declared. Fix the
code accidentally relying on <sys/stat.h> including <sys/types.h>.

Finally fix <dirent.h> pulling in <stdint.h> through <sortix/dirent.h>.

The <sortix/__/dt.h> and <sortix/__/stat.h> headers are no longer required
and their contents have been merged into <sortix/__/dirent.h>.

Fix -C disabling checking rather than checking quietly.

Fix sort(1) exiting 1 on certain errors, as POSIX requires sort(1) to only
exit if the input wasn't sorted when -c.

Fix -o opening the output file for truncation before all the input has been
read, as POSIX requires allowing -o to be an input file.

POSIX requires sort(1) to handle input errors by either erroring with no
output, or by erroring and sorting the input read so far. Change the current
behavior of continuing to the next file to simply failing hard on the first
input error.

Don't increment the last line number on the end of the standard input.

Report -c/-C as incompatible with -o.

Exit unsuccessfully on any output errors.

Update to current coding conventions and add documentation while here.