Add TODO and SUS-compliant yes.

TODO

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+Write a Makefile.
+Add octal escape support to echo.
+Rework yes(1) to buffer input. This achieves greater throughput per system call.
+
+Write the following utilities:
+ ar
+ at/atd
+ basename
+ batch(?)
+ bc
+ cal
+ cat
+ chgrp
+ chmod
+ chownn
+ cksum
+ cmp
+ comm
+ compress(?)
+ cp
+ crond/crontab
+ csplit
+ cut
+ dd
+ df
+ diff
+ dirname
+ du
+ ed/x
+ expand(?)
+ expr
+ file
+ find
+ fold
+ fuser
+ grep
+ head
+ iconv
+ id
+ join
+ kill
+ link(?)
+ ln
+ locale
+ localedef
+ logname
+ lp(?)
+ ls
+ mesg
+ mkdir
+ mkfifo
+ mknod
+ mktemp
+ more
+ mv
+ newgrp(?)
+ nice
+ nl
+ paste
+ pathchk(?)
+ pax
+ printf
+ ps
+ pwd
+ renice
+ rm
+ rmdir
+ sed
+ sh
+ sleep
+ sort
+ split
+ strings
+ tabs
+ tail
+ talk(?)
+ tee
+ test
+ touch
+ tput
+ tr
+ true
+ tsort
+ uname
+ uncompress(?)
+ unexpand
+ uniq
+ wc
+ who
+ write(?)
+ xargs

doc/echo.1

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+.TH echo 1 2021-11-21 mp-utils Userland
+
+.SH NAME
+echo \- Print argv to stdout.
+.PP
+.SH SYNOPSIS
+.PP
+Print the provided arguments to stdout while applying
+.SM
+SUS
+backslash escapes.
+.SH DESCRIPTION
+.PP
+Echo is a standard Unix tool that accepts input from the user and prints it to stdout while applying minimal or no formatting.
+.SH OPTIONS
+.PP
+This implementation of echo accepts no arguments.
+.SH ENVIRONMENT
+This implementation of echo requires a C89 compiler.
+.SH FILES
+/usr/src/mp-utils/src/echo.c
+.SH CONFORMING TO
+.PP
+.SM
+POSIX
+2017,
+.SM
+SUS
+3
+.SH NOTES
+.PP
+Including the following backslash-escapes modifies echo's behavior.
+
+\\a \- Print a BEL.
+
+\\b \- Print a backspace.
+
+\\f \- Print a form-feed.
+
+\\f \- Print a newline.
+
+\\r \- Print a carriage return.
+
+\\t \- Print a tabulation.
+
+\\v \- Print a vertical tabulation.
+
+\\c \- Suppress the trailing newline and ignore further input.
+
+.SH BUGS
+.PP
+Currently, \\0 (octal) escapes have not been implemented.
+.SH SEE ALSO
+echo(1p), printf(1)

doc/yes.1

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+.TH yes 1 2021-11-21 mp-utils Userland
+
+.SH NAME
+yes \- print to stdout forever
+.PP
+.SH SYNOPSIS
+.PP
+Print the provided string to stdout forever.
+.SH DESCRIPTION
+.PP
+Yes is a standard Unix tool that accepts input from the user and prints it to stdout repeatedly, or y when no input is provided.
+.SH OPTIONS
+.PP
+This implementation of yes accepts no arguments.
+.SH ENVIRONMENT
+This implementation of yes requires a C89 compiler.
+.SH FILES
+/usr/src/mp-utils/src/yes.c
+.SH CONFORMING TO
+.PP
+.SM
+POSIX
+2017,
+.SM
+SUS
+3
+.SH NOTES
+.PP
+None.
+.SH BUGS
+.PP
+None known.
+.SH SEE ALSO
+yes(1p)

src/yes.c

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+#include "common.h"
+
+int main(int argc, char **argv)
+    {
+    if(argc > 1)
+	while(1)
+	    puts(argv[1]);
+    else
+	while(1)
+	    puts("y");
+
+    return 0;
+    }