getgrnam, getgrgid, getpwnam, getpwuid for host

The functions now read /etc/group and /etc/passwd on host machines.
Android-specific IDs are not recognized in the host.

getpwent and getgrent are still not working though. They require a
bigger refactoring to sequentially advance in the database files.

Bug: 171718702
Test: run assemble_cvd in aosp_cf_arm64_phone
Change-Id: Ie8da382a467bbd0bffac7b4b8592cd871db80181
diff --git a/libc/bionic/grp_pwd_file.h b/libc/bionic/grp_pwd_file.h
index 69c771b..5fd3d2f 100644
--- a/libc/bionic/grp_pwd_file.h
+++ b/libc/bionic/grp_pwd_file.h
@@ -65,7 +65,10 @@
   const char* filename_ = nullptr;
   const char* start_ = nullptr;
   const char* end_ = nullptr;
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunused-private-field"
   const char* required_prefix_;
+#pragma clang diagnostic pop
 };
 
 class PasswdFile {