Perl_sv_vcatpvfn_flags: simpler special formats

Perl_sv_vcatpvfn_flags: simpler special formats

At the top of Perl_sv_vcatpvfn_flags(), certain fixed formats are
special-cased: "", "%s", "%-p", "%.0f".

Simplify the code which handles these. In particular, don't try to issue
"missing" or "redundant" arg warnings there. Instead, check for the
correct number of args as part of the test for whether this can be
special-cased, and if not, fall through to the general code in the main
body of the function to handle that format and issue any warnings.

This makes the code a lot simpler. It also now detects the redundant arg
in printf("%.0f",1,2).

The code is now also more efficient - it tries to check for things like
pat[0] == '%' only once, rather than re-checking for every special-case
variant its trying.