LibCore: Reset `errno` before using LibC pwd functions

We should not expect LibC functions to clear `errno` on success,
so if we want to use it for error checking after a call, we need
to clear it before the call.

Userland/Libraries/LibCore/System.cpp

@@ -404,6 +404,8 @@ ErrorOr<void> chown(StringView pathname, uid_t uid, gid_t gid)
 
 ErrorOr<Optional<struct passwd>> getpwnam(StringView name)
 {
+    errno = 0;
+
     ::setpwent();
     if (errno)
         return Error::from_syscall("getpwnam"sv, -errno);
@@ -422,6 +424,8 @@ ErrorOr<Optional<struct passwd>> getpwnam(StringView name)
 
 ErrorOr<Optional<struct group>> getgrnam(StringView name)
 {
+    errno = 0;
+
     ::setgrent();
     if (errno)
         return Error::from_syscall("getgrnam"sv, -errno);