Use tor_getpw{nam,uid} wrappers to fix bug 11946

When running with User set, we frequently try to look up our
information in the user database (e.g., /etc/passwd).  The seccomp2
sandbox setup doesn't let us open /etc/passwd, and probably
shouldn't.

To fix this, we have a pair of wrappers for getpwnam and getpwuid.
When a real call to getpwnam or getpwuid fails, they fall back to a
cached value, if the uid/gid matches.

(Granting access to /etc/passwd isn't possible with the way we
handle opening files through the sandbox.  It's not desirable either.)