LibC+Tests: Simplify getpwuid_r() and getpwnam_r() and add tests

These functions are now implemented in terms of getpwent_r() which
allows us to remove two FIXMEs about global variable shenanigans.

I'm also adding tests for both APIs. :^)

Tests/LibC/CMakeLists.txt

@@ -20,6 +20,7 @@ set(TEST_SOURCES
     TestPThreadPriority.cpp
     TestPthreadSpinLocks.cpp
     TestPthreadRWLocks.cpp
+    TestPwd.cpp
     TestQsort.cpp
     TestRaise.cpp
     TestRealpath.cpp