pbx: Create pbx_sw.c for management of 'struct ast_sw'.

This changes context switches from a linked list to a vector, makes
'struct ast_sw' opaque to pbx.c.

Although ast_walk_context_switches 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_switches_count (AST_VECTOR_SIZE)
* ast_context_switches_get (AST_VECTOR_GET)

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

Change-Id: I08deb016df22eee8288eb03de62593e45a1f0998