libsframe: refactor code for dumping section flags

libsframe: refactor code for dumping section flags

To prepare code for accommodating new flag additions easily as the
format evolves.

libsframe/
        * sframe-dump.c (SFRAME_HEADER_FLAGS_STR_MAX_LEN): Rename from.
        (struct dump_flags_helper): New helper struct definition.
        (dump_sframe_header_flags): .. to here.  New definition.
        (SFRAME_FLAGS_STR_MAX_LEN): Rename to.
        (MAX_NUM_FLAGS): New definition.
        (dump_sframe_header): Move some implementation from here ..