[CM] libs7: inconsist naming of s7 optimizing registration

[bil at ccrma.Stanford.EDU]

The short answer is that it's an historical artifact, and
I try not to change s7.h unless it's absolutely necessary.

The long form: the s7_p_* procedures came first when I was
developing the second level of the optimizer, and at that
time I thought I'd always need the s7_scheme* argument, so
there was no need to mention it.  I later added the s7_i_*
procedures (without the "7") as the optimizer grew.  Then
I ran into cases where the s7_i_* procedures needed a version
that included the s7_scheme* argument, so I added the "7" to
distinguish the two cases.  The s7_p_* procedures always
take the s7_scheme* argument, so I view it as redundant to
insist on it in the name.

I think s7.h stability very greatly outweighs almost any
other consideration, including name consistency.