partially freeing up some SV flags

This series of commits I've just pushed eliminate some obscure uses of
various SV flags:

    a5c7cb0 eliminate SVpbm_VALID flag
    b4204fb eliminate SVpbm_TAIL/SvTAIL_on()/SvTAIL_off()
    e08d24f Only test SvTAIL when SvVALID
    e068d7c S_setup_longest(): SvTAIL() used where always 0
    6432a58 Eliminate SVrepl_EVAL and SvEVALED()
    9a70c74 remove DOES's usage of SvSCREAM
    4c57ced remove eval's usage of SvSCREAM


After these commits,

 0x00008000 is now only used on HVs, GVs and ROK scalars;
 0x40000000 is now only used on AVs and HVs;
 0x80000000 is now only used on AVs, HVs, ROK and IOK scalars.


-- 
The Enterprise is captured by a vastly superior alien intelligence which
does not put them on trial.
    -- Things That Never Happen in "Star Trek" #10

• partially freeing up some SV flags by Dave Mitchell