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