LibCore: Add File::OpenMode::DontCreate

Some applications may not want to have the ability to create a
file if it doesn't exist, but still be able to read and write
from it. The easy solution here would be just to not apply
O_CREAT when creating the flags, but to prevent breaking a ton
of applications, having a `DontCreate` mode is the best for now.

Userland/Libraries/LibCore/File.cpp

@@ -87,6 +87,13 @@ int File::open_mode_to_options(OpenMode mode)
         flags |= O_CLOEXEC;
     if (!has_flag(mode, OpenMode::Nonblocking))
         flags |= O_NONBLOCK;
+
+    // Some open modes, like `ReadWrite` imply the ability to create the file if it doesn't exist.
+    // Certain applications may not want this privledge, and for compability reasons, this is
+    // the easiest way to add this option.
+    if (has_flag(mode, OpenMode::DontCreate))
+        flags &= ~O_CREAT;
+
     return flags;
 }
 

Userland/Libraries/LibCore/File.h

@@ -31,6 +31,7 @@ public:
         MustBeNew = 16,
         KeepOnExec = 32,
         Nonblocking = 64,
+        DontCreate = 128,
     };
 
     enum class ShouldCloseFileDescriptor {