* include/libmes.h (dtoab): Declare.
* lib/mes/ntoab.c (ntoab): Update.
* lib/mes/dtoab.c: New file.
* build-aux/configure-lib.sh (libmes_SOURCES, libc_tcc_SOURCES): Add
it.
* lib/tests/mes/90-dtoab.c: Test it.
* lib/tests/stdio/90-sprintf.c: Test it.
* build-aux/check-mescc.sh (tests): Run it, against...
* lib/tests/mes/90-dtoab.stdout: ...baseline.
* lib/stdio/vfprintf.c (vfprintf): Support %f, even more naive support
for %e, %E, %g, %G.
* lib/stdio/vsnprintf.c (vsnprintf): Likewise.
