as an extension, have putenv("VAR") behave as unsetenv("VAR")

the behavior of putenv is left undefined if the argument does not
contain an equal sign, but traditional implementations behave this way
and gnulib replaces putenv if it doesn't do this.

src/env/putenv.c

@@ -9,14 +9,14 @@ char **__env_map;
 int __putenv(char *s, int a)
 {
 	int i=0, j=0;
-	char *end = strchr(s, '=');
-	size_t l = end-s+1;
+	char *z = strchr(s, '=');
 	char **newenv = 0;
 	char **newmap = 0;
 	static char **oldenv;
-	
-	if (!end || l == 1) return -1;
-	for (; __environ[i] && memcmp(s, __environ[i], l); i++);
+
+	if (!z) return unsetenv(s);
+	if (z==s) return -1;
+	for (; __environ[i] && memcmp(s, __environ[i], z-s+1); i++);
 	if (a) {
 		if (!__env_map) {
 			__env_map = calloc(2, sizeof(char *));