(info): Don't leak potentially sensitive information to player in

(info): Don't leak potentially sensitive information to player in
error messages.  Properly log all system errors and return RET_SYS.
Closes #1028552.

(info): Fix use of uninitialized variable when fopen() succeeds but
fstat() or the st_mode check fails.

(info) [_WIN32]: Fix '//' to be '\\'.  Add file type check to prevent
opening hidden or operating system files.