fix up environ array so subsequent getenv() calls will return the
modified value.  _envsetup() is not appropriate here because of
the extra work it does which is not wanted.

Reference: /n/atom/patch/applied2013/apeputenv
Date: Wed Aug 21 01:33:20 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/ape/lib/bsd/putenv.c	Wed Aug 21 01:32:28 2013
+++ /sys/src/ape/lib/bsd/putenv.c	Wed Aug 21 01:32:28 2013
@@ -1,8 +1,56 @@
+#include <stdlib.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <string.h>
 
+extern char **environ;
+
+static int
+envredo(char *s)
+{
+	char *x, **p, *q, **v, *mem;
+	int n, nsz, nenv, esz;
+
+	x = strchr(s, '=');
+	if(x == NULL)
+		return -1;
+	nsz = x - s + 1;		/* compare the var=, not just var */
+
+	nenv = 1;
+	esz = strlen(s)+1;
+	for(p = environ; (q = *p) != NULL; p++){
+		esz += strlen(q)+1;
+		nenv++;
+	}
+
+	/* overestimate in the case we have changed a variable. */
+	v = malloc((nenv+1)*sizeof(char**) + esz);
+	if(v == NULL)
+		return -1;
+	mem = (char*)(v + nenv + 1);
+
+	nenv = 0;
+	for(p = environ; (q = *p) != NULL; p++){
+		if(strncmp(q, s, nsz) == 0)
+			continue;
+		v[nenv++] = mem;
+		n = strlen(q)+1;
+		memcpy(mem, q, n);
+		mem += n;
+	}
+	v[nenv++] = mem;
+	n = strlen(s)+1;
+	memcpy(mem, s, n);
+
+	v[nenv] = NULL;
+
+	free(environ);
+	environ = v;
+
+	return 0;
+}
+
 int
 putenv(char *s)
 {
@@ -26,7 +74,7 @@
 		if(write(f, value, n) != n)
 			return -1;
 		close(f);
-		return 0;
+		return envredo(s);
 	} else
 		return -1;
 }