Link callback arrays become static variables

Since they are used only in the context of the string_process_bug_link()
and string_process_bugnote_link() functions, we do not actually need to
use global variables; local, static ones make more sense.

• $g_string_process_bug_link_callback -> $s_bug_link_callback
• $g_string_process_bugnote_link_callback -> $s_bugnote_link_callback