We need to be removing things from the static interface, not adding to 
it. During my first attempt to implement user management in libsepol, I 
added a few functions that work with a context_struct. Some are new 
functions that I wrote to convert a record to a context_struct, others I 
pulled out of services.c to organize into context.c.

They were prefixed sepol_ctx_struct_*.

This patch:
- deletes those functions from the static interface, and puts them in a 
private header. Those are the first of (hopefully more to come) 
functions to be drawn from the static interface (as described by 
published headers) back into libsepol where they belong.
- removes the sepol_ prefix, which I use to indicate whether a function 
is exported or not
- changes "ctx_struct" to "context" for consistency with other code that 
uses a context_struct (context_cpy, etc..)
- removes unnecessary relay function from services.c, and use the right 
function directly.
- renames the create() function to _from_record(), which describes more 
accurately what is going on.
- replaces all inclusion of <sepol/policydb/context.h> with the internal 
"context.h", which draws in the other header

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Note: A possible point of confusion - the internal context functions are 
now all prefixed context_*. The record ones are prefixed 
(sepol_context_*). They both represent a context, but the record is 
exported via the shared interface. The internal structure is only 
exported via the static interface.