system is a cancellation point

ideally, system would also be cancellable while running the external
command, but I cannot find any way to make that work without either
leaking zombie processes or introducing behavior that is far outside
what the standard specifies. glibc handles cancellation by killing the
child process with SIGKILL, but this could be unsafe in that it could
leave the data being manipulated by the command in an inconsistent
state.

src/process/system.c

@@ -13,6 +13,7 @@ weak_alias(dummy_0, __acquire_ptc);
 weak_alias(dummy_0, __release_ptc);
 
 pid_t __vfork(void);
+void __testcancel(void);
 
 int system(const char *cmd)
 {
@@ -21,6 +22,8 @@ int system(const char *cmd)
 	struct sigaction sa = { .sa_handler = SIG_IGN }, oldint, oldquit;
 	int status = -1, i;
 
+	__testcancel();
+
 	if (!cmd) return 1;
 
 	sigaction(SIGINT, &sa, &oldint);