CLSA-2026-1777476716 vim: Fix of 8 CVEs

CVE-2021-4019: replace the unbounded STRCPY pair in findhelptags with vimsnprintf bounded by IOSIZE to prevent heap buffer overflow with long :help arguments starting with "%z@". - CVE-2021-4192: re-fetch regline/reginput via reggetline after getvvcol in regmatchvisual so the cached line pointer...