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.mk b/libc/Android.mk
index 0e4dc11..49a8bdb 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -115,6 +115,7 @@
     bionic/getpid.cpp \
     bionic/gettid.cpp \
     bionic/__gnu_basename.cpp \
+    bionic/grp_pwd.cpp \
     bionic/ifaddrs.cpp \
     bionic/inotify_init.cpp \
     bionic/ioctl.cpp \