Note interactive shell errors for make buildenv and add a warning for

people tempted to add back the || true to get rid of them.

Sponsored by: Netflix

Makefile.inc1

@@ -988,6 +988,14 @@ buildenvvars: .PHONY
 .endif
 .endif
 BUILDENV_DIR?=	${.CURDIR}
+#
+# Note: make will report any errors the shell reports. This can
+# be odd if the last command in an interactive shell generates an
+# error or is terminated by SIGINT. These reported errors look bad,
+# but are harmless. Allowing them also allows BUIDLENV_SHELL to
+# be a complex command whose status will be returned to the caller.
+# Some scripts in tools rely on this behavior to report build errors.
+#
 buildenv: .PHONY
 	@echo Entering world for ${TARGET_ARCH}:${TARGET}
 .if ${BUILDENV_SHELL:M*zsh*}