pbx: Create pbx_ignorepat.c for management of 'struct ast_ignorepat'.

This changes context ignore patterns from a linked list to a vector,
makes 'struct ast_ignorepat' opaque to pbx.c.

Although ast_walk_context_ignorepats is maintained the procedure is no
longer efficient except for the first call (inc==NULL).  This
functionality is replaced by two new functions implemented by vector
macros.
* ast_context_ignorepats_count (AST_VECTOR_SIZE)
* ast_context_ignorepats_get (AST_VECTOR_GET)

As with ast_walk_context_ignorepats callers of these functions are
expected to have locked contexts.  Only a few places in Asterisk walked
the ignorepats, they have been converted to use the new functions.

Change-Id: I78f2157d275ef1b7d624b4ff7d770d38e5d7f20a