Add limited support for printing floating point numbers to
VG_(debugLog_vprintf). 
Remove function VG_(percentify) and fix up its call sites (part of
fixing BZ #337869.
Allow the width in a format specification to be '*', i.e. the width is
given as an additional function argument.

The limitations for printing floating point numbers are:
(1) %f is the only supported format. Width and precision can be
    specified.
(2) Funny numbers (NaN and such) are not supported.
(3) Floating point numbers need to be benign in the sense that their
    integral part fits into an ULong.
This is good enough for our purposes.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14806 a5019735-40e9-0310-863c-91ae7b9d1cf9