[libc] Add getpwent, setpwent, and endpwent entrypoints (#213076)

Added getpwent, setpwent, and endpwent functions using the internal
pwd_utils line parser.

* Implemented getpwent.cpp, setpwent.cpp, and endpwent.cpp entrypoints
* Added database iteration state and helpers (open, close, read_next)
under namespace passwd in pwd_utils
* Registered entrypoints in config/linux/*/entrypoints.txt
* Added CMake target guards for File and platform_file to support
overlay builds and cross-platform targets
* Added unit tests in libc/test/src/pwd/getpwent_test.cpp

Assisted-by: Automated tooling, human reviewed.
GitOrigin-RevId: 096481ddc5aa696f26ae4fd704adb28ce31babc6