regedit: Improvements to importing files

Minor enhancements to the regedit tool.

Change Log:

• Put a new line character in IDS_IMPORT_PROMPT in order to try to make the dialog a bit smaller in width.
• In wWinMain(), when ProcessCmdLine() returns TRUE, change the return value from 0 to EXIT_SUCCESS to increase readability.
• When importing registry files from the command line and a dialog box appears asking the user if they want to import a registry file, I changed ID_YESNO to ID_YESNOCANCEL to help increase flexibility. The use case for this new change is, for example, if the user decides to import more than one registry file at once, but then decides last minute that they want to cancel out of the operation. This saves the user from the trouble of either saying no to every dialog box (especially if there is a lot of registry files) or having to end the program process manually.

The code compiles without any errors or warnings. But it needs to be properly tested before it's ready.