[libcalamares] Add mode to createTargetFile()

- Unconditionally **not** overwriting the target file isn't an option:
  writing hostname, for instance, expects that to be done even
  if `/etc/hostname` already exists on the target filesystem.

src/libcalamares/utils/CalamaresUtilsSystem.cpp

@@ -247,7 +247,7 @@ System::targetPath( const QString& path ) const
 }
 
 CreationResult
-System::createTargetFile( const QString& path, const QByteArray& contents ) const
+System::createTargetFile( const QString& path, const QByteArray& contents, WriteMode mode ) const
 {
     QString completePath = targetPath( path );
     if ( completePath.isEmpty() )
@@ -256,7 +256,7 @@ System::createTargetFile( const QString& path, const QByteArray& contents ) cons
     }
 
     QFile f( completePath );
-    if ( f.exists() )
+    if ( ( mode == WriteMode::KeepExisting ) && f.exists() )
     {
         return CreationResult( CreationResult::Code::AlreadyExists );
     }
@@ -264,7 +264,7 @@ System::createTargetFile( const QString& path, const QByteArray& contents ) cons
     QIODevice::OpenMode m =
 #if QT_VERSION >= QT_VERSION_CHECK( 5, 11, 0 )
         // New flag from Qt 5.11, implies WriteOnly
-        QIODevice::NewOnly |
+        ( mode == WriteMode::KeepExisting ? QIODevice::NewOnly : QIODevice::WriteOnly ) |
 #endif
         QIODevice::WriteOnly | QIODevice::Truncate;
 

src/libcalamares/utils/CalamaresUtilsSystem.h

@@ -258,6 +258,12 @@ public:
      */
     DLLEXPORT QString targetPath( const QString& path ) const;
 
+    enum class WriteMode
+    {
+        KeepExisting,
+        Overwrite
+    };
+
     /** @brief Create a (small-ish) file in the target system.
      *
      * @param path Path to the file; this is interpreted
@@ -265,14 +271,18 @@ public:
      *      but / in the chroot, or / in OEM modes).
      * @param contents Actual content of the file.
      *
-     * Will not overwrite files. Returns an empty string if the
-     * target file already exists.
+     * If the target already exists:
+     *  - returns AlreadyExists as a result (and does not overwrite),
+     *  - **unless** @p mode is set to Overwrite, then it tries writing as
+     *    usual and will not return AlreadyExists.
      *
      * @return The complete canonical path to the target file from the
      *      root of the host system, or empty on failure. (Here, it is
      *      possible to be canonical because the file exists).
      */
-    DLLEXPORT CreationResult createTargetFile( const QString& path, const QByteArray& contents ) const;
+    DLLEXPORT CreationResult createTargetFile( const QString& path,
+                                               const QByteArray& contents,
+                                               WriteMode mode = WriteMode::KeepExisting ) const;
 
     /** @brief Remove a file from the target system.
      *