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rc: flush environment variables (update /env) before fork

on races... normal forks will all share the /env environment but
not the in memory variables of rc. so when we would normally fork
whoever does an exec (flush) first will override what the values of the
/env variables are, *independent* of the variables that where
actually modified *in* the process.

when we flush *before* fork, then at least both processes start out
with marked clean in memory variables and the processes will flush
only the things they actually change.

--- a/sys/src/cmd/rc/havefork.c

+++ b/sys/src/cmd/rc/havefork.c

@@ -12,10 +12,12 @@

 	int null = open("/dev/null", 0);

 	int pid;

 	char npid[10];

+

 	if(null<0){

 		Xerror("Can't open /dev/null\n");

 		return;

 	}

+	Updenv();

 	switch(pid = rfork(RFFDG|RFPROC|RFNOTEG)){

 	case -1:

 		close(null);

@@ -45,10 +47,12 @@

 	int lfd = p->code[pc++].i;

 	int rfd = p->code[pc++].i;

 	int pfd[2];

+

 	if(pipe(pfd)<0){

 		Xerror("can't get pipe");

 		return;

 	}

+	Updenv();

 	switch(forkid = fork()){

 	case -1:

 		Xerror("try again");

@@ -91,6 +95,7 @@

 		Xerror("can't make pipe");

 		return;

 	}

+	Updenv();

 	switch(pid = fork()){

 	case -1:

 		Xerror("try again");

@@ -152,6 +157,7 @@

 	char name[40];

 	int pfd[2];

 	int sidefd, mainfd;

+

 	if(pipe(pfd)<0){

 		Xerror("can't get pipe");

 		return;

@@ -164,6 +170,7 @@

 		sidefd = pfd[PRD];

 		mainfd = pfd[PWR];

 	}

+	Updenv();

 	switch(pid = fork()){

 	case -1:

 		Xerror("try again");

@@ -191,6 +198,8 @@

 Xsubshell(void)

 {

 	int pid;

+

+	Updenv();

 	switch(pid = fork()){

 	case -1:

 		Xerror("try again");

-- 

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