Override `non_shake_case' warning on per-function basis

This change removes global override of `non_shake_case' warning and replaces it
with per-function overrrides. This way, compiler will complain about style
guide violation in new code.

It should be noted, that `rustc' is not smart enough to emit warning when
override is no longer needed, it must be checked manually.

src/dc_configure.rs

@@ -99,6 +99,7 @@ pub fn dc_stop_ongoing_process(context: &Context) {
 }
 
 // the other dc_job_do_DC_JOB_*() functions are declared static in the c-file
+#[allow(non_snake_case)]
 pub unsafe fn dc_job_do_DC_JOB_CONFIGURE_IMAP(context: &Context, _job: *mut dc_job_t) {
     let flags: libc::c_int;
     let mut current_block: u64;