os: Don't use legacy getwd() libc function, not available in all libc's.

For example, not available in uclibc.

os/metadata.txt

@@ -1,5 +1,5 @@
 srctype = micropython-lib
 type = package
-version = 0.1.6
+version = 0.1.7
 author = Paul Sokolovsky
 depends = libc, errno, stat

os/os/__init__.py

@@ -18,7 +18,7 @@ mkdir_ = libc.func("i", "mkdir", "si")
 rename_ = libc.func("i", "rename", "ss")
 unlink_ = libc.func("i", "unlink", "s")
 rmdir_ = libc.func("i", "rmdir", "s")
-getwd_ = libc.func("s", "getwd", "s")
+getcwd_ = libc.func("s", "getcwd", "si")
 opendir_ = libc.func("P", "opendir", "s")
 readdir_ = libc.func("P", "readdir", "P")
 read_ = libc.func("i", "read", "ipi")
@@ -56,7 +56,7 @@ def raise_error():
 
 def getcwd():
     buf = bytearray(512)
-    return getwd_(buf)
+    return getcwd_(buf, 512)
 
 def mkdir(name, mode=0o777):
     e = mkdir_(name, mode)

os/setup.py

@@ -6,7 +6,7 @@ from setuptools import setup
 
 
 setup(name='micropython-os',
-      version='0.1.6',
+      version='0.1.7',
       description='os module for MicroPython',
       long_description="This is a module reimplemented specifically for MicroPython standard library,\nwith efficient and lean design in mind. Note that this module is likely work\nin progress and likely supports just a subset of CPython's corresponding\nmodule. Please help with the development if you are interested in this\nmodule.",
       url='https://github.com/micropython/micropython/issues/405',