Feature request: option to mark function as safe

External functions are assumed to be unsafe (https://doc.rust-lang.org/reference/items/external-blocks.html#r-items.extern.fn.safety) but there should be an option to mark one as safe

For example, for FFI bindings for the PHP source, the function zend_html_putc (https://github.com/php/php-src/blob/d03163dc0af54f9c0687ab345f5db2584563b2a7/Zend/zend_highlight.c#L28) is always going to be safe - it just prints a character. Similarly, zend_gdb_present (https://github.com/php/php-src/blob/PHP-8.5.10/Zend/zend_gdb.c#L110) takes no parameters and returns a boolean reporting if GDB is present - this function is always going to be safe to call

There should be a way to indicate that a specific function should be marked as safe

Start by tracing bindgen's handling of external functions and its configuration options. Check how an explicit safe marker could be represented for selected functions, and add coverage for the PHP examples or equivalent generated bindings. Done means users can mark specific external functions as safe without changing the default for other externals.