sv.c: Remove reference to bogus SvNUM macro

There has never been such a macro.
sv.c: Note that sv_2num does not do get-magic
Don’t call string overloading in chr

See <20141130160250.GC31019@pjcj.net>.  Commit 354b74ae6f broke this.
Don’t do string overloading for printf int formats

See <20141130160250.GC31019@pjcj.net>.  Commit 354b74ae6f broke this.
Don’t do string overloading for numeric pack fmts

See <20141130160250.GC31019@pjcj.net>.  Commit 354b74ae6f broke this.