include: libsframe: add APIs for SFrame header flags

include: libsframe: add APIs for SFrame header flags

Add new APIs, one each for getting flags from the SFrame decoder and
SFrame encoder context objects respectively.

These will later be used by the linker to uniformly access the flags,
given the SFrame decoder and SFrame encoder objects.

Use the new API, where applicable, within libsframe.

include/
        * sframe-api.h (sframe_decoder_get_flags): New declaration.
        (sframe_encoder_get_flags): Likewise.
libsframe/
	* libsframe.ver: List new APIs.
        * sframe.c (sframe_decoder_get_flags): New definition.
	(sframe_encoder_get_flags): Likewise.
        (sframe_get_funcdesc_with_addr_internal): Use the new API.
        (sframe_encoder_get_flags): Likewise.
        (sframe_encoder_write_sframe): Likewise.