Revised types for functions cs_complete_auto, etc.

Revised types for functions cs_complete_auto, etc.

The functions cs_complete_auto, cs_complete_auto_head takes 'const char *'
as buffer rather than 'const unsigned char *'. This makes some casts
unnecessary. This also fixes tst_comstack which used to test
cs_complete_http. This function, howver is no longer public, so
cs_complete_auto is used instead.