util: add functions for common file operations

util: add functions for common file operations

Add a function to open a file for reading, writing, or appending.
In uppercase modes errors are handled as fatal, i.e. the caller doesn't
need to check for NULL. To avoid string manipulations in the callers,
the function accepts an optional directory and suffix. New files are
created with specified permissions, which will be needed for saving
keys. The O_EXCL flag is used in the writing mode to make sure a new
file is created (on filesystems that support it).

Also, add a function to rename a temporary file by changing its suffix,
and a function to remove a file.

All functions log all errors, at least as debug messages.