bionic: split out getpwnam and friends

- added grp_pwd.cpp containing POSIX passwd and group functions,
  colocated because they share with the Android ID (AID) roots.
- stubs.cpp contains all the truly empty functions (network and
  protocol accessors)

Bug: 27999086
Change-Id: I036f9e2dd246f48302cb7c97d23176fa24d19c33
diff --git a/libc/Android.bp b/libc/Android.bp
index cbca35f..de0ac0d 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1286,6 +1286,7 @@
         "bionic/getpid.cpp",
         "bionic/gettid.cpp",
         "bionic/__gnu_basename.cpp",
+        "bionic/grp_pwd.cpp",
         "bionic/ifaddrs.cpp",
         "bionic/inotify_init.cpp",
         "bionic/ioctl.cpp",