Fix C++ compiler error in monster1.c

Fix parameter on file_open().
Fix C++ compiler errors in obj-ui.c.

Add required cast and return correct type.
Fix C++ compiler errors in prefs.c.

Required cast and file_open() parameter.
Replace C++ reserved keywords in spells1.c.

Replace uses of 'protected'.
Fix C++ compiler errors in squelch.c.

Add required casts.