numfmt: implement --format

[cakebaker]

This PR implements --format:

Use printf-style floating FORMAT string. The format string must contain one ‘%f’ directive, optionally with ‘'’, ‘-’, ‘0’, width or precision modifiers. The ‘'’ modifier will enable --grouping, the ‘-’ modifier will enable left-aligned --padding and the width modifier will enable right-aligned --padding. The ‘0’ width modifier (without the ‘-’ modifier) will generate leading zeros on the number, up to the specified width.

While the parsing of the format string is fully implemented, the handling of the ' modifier (grouping) is not implemented and hence has no effect on the output.

This PR makes all precision, fmt, and fmt-err (with the exception of fmt-err-9 and fmt-err-11) tests in https://github.com/coreutils/coreutils/blob/master/tests/misc/numfmt.pl pass.

[sylvestre]

Impressive work.
Do you know if the code could be useful in other coreutils binaries?

[cakebaker]

I don't know, the code is probably too specific to numfmt.

[sylvestre]

OK merci :)