chgserver: do not send system() back to client if stdio redirected (issue5992) As the chg client doesn't know server-side stdio redirection, the server shouldn't upcall on "runsystem" request if the stdio streams are redirected. This patch teaches ui to remember the redirection flag, which is updated by the caller right now. Future patches (for default) will add ui methods to manage this flag internally.

chgserver: update comment describing when to fall back to core _runsystem() The condition "output needs to be captured" was moved to the core ui, and removed at fbce78c58f1e "chg: refactor ui.system() to be partly overridden." The next patch will add one more "if" to address the issue 5992.