Reinstate "regcomp.c: Make inversion lists SVt_PVLV"

Reinstate "regcomp.c: Make inversion lists SVt_PVLV"

This reverts commit ac7a6f5849b5fd57b2e837f4ddbc18c992610e9c which
reverted 2c3365de8c1168f115576a4976d067e3b911c490, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

This commit additionally changes some now-obsolete wording in a pod.
This change was not in 2c3365de8c1168f115576a4976d067e3b911c490.

The original message for commit 2c3365de8c1168f115576a4976d067e3b911c490
was:

This is the 2nd step in separating the inversion list body from header.
This commit gives inversion lists the header from a SVt_PVLV, and
repurposes one of its fields into being the length of the inversion
list.

This is a temporary measure, in case binary compatibility is an issue.
Future commits will create a new SV type just for inversion lists.  This
SV type was chosen because it has a sufficient number of fields to
accommodate all the header fields from inversion lists.