os: Implement getenv().

Author: pfalcon

os/example_getenv.py

@@ -0,0 +1,3 @@
+import os
+
+print(os.getenv("HOME", "def"))

os/os/__init__.py

@@ -32,6 +32,7 @@
 getpid_ = libc.func("i", "getpid", "")
 waitpid_ = libc.func("i", "waitpid", "ipi")
 system_ = libc.func("i", "system", "s")
+getenv_ = libc.func("s", "getenv", "P")
 
 R_OK = const(4)
 W_OK = const(2)
@@ -204,6 +205,12 @@ def system(command):
     check_error(r)
     return r
 
+def getenv(var, default=None):
+    var = getenv_(var)
+    if var is None:
+        return default
+    return var
+
 def fsencode(s):
     if type(s) is bytes:
         return s