MFC r248870 by joel: Minor mdoc fix in previous commit.
MFC r251763: sh(1): A subshell environment has its own rlimits (ulimit).

This has always been the case and is intended (just like cd).

This matches Austin group issue #706.
MFC r245383,245392,247190,249220,251180,251797: New sh testcases.

These already work on stable/9.
MFC r248349: sh: Recognize "--" and explicitly reject options in wait
builtin.

If syntactically invalid job identifiers are to be taken as jobs that exited
with status 127, this should not apply to options, so that we can add
options later if need be.
MFC r248774: accept(2): Mention inheritance of O_ASYNC and signal
destination.

While almost nobody uses O_ASYNC, and rightly so, the inheritance of the
related properties across accept() is a portability issue like the
inheritance of O_NONBLOCK.
MFC r251181: fork(2): #include <sys/types.h> is not needed.