MFC r262755: sh: Make argstr() return where it stopped and simplify expari()
using this.
MFC r263195: sh: Add some consts.
MFC r263244: find: When performing -quit, finish pending -exec ... + command
lines.

This avoids unexpected partial processing when a find command uses both
-quit and -exec ... +.

GNU find does the same.
waitid(2): Do not tell userland programmers to include <sys/signal.h>.

Userland should get these definitions by including <signal.h>.
MFC r258794: chmod(2): Document S_ISVTX following SUSv3/SUSv4.

S_ISTXT is non-standard.

While here, also update fchmodat() standards entry to POSIX.1-2008.