qapi: Rename identical c_fun()/c_var() into c_name()

qapi: Rename identical c_fun()/c_var() into c_name()

Now that the two functions are identical, we only need one of them,
and we might as well give it a more descriptive name.  Basically,
the function serves as the translation from a QAPI name into a
(portion of a) C identifier, without regards to whether it is a
variable or function name.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>