quotearg: add quotearg_n_style_colon() This quotes with default options of the specified style, but with quoting enabled for instances of ':'. While this can be simulated by temporarily modifying the default quoting style, this is a little awkward, and care must be taken with such an implementation to avoid inlining leading to bloated text. * lib/quotearg.h (quotearg_n_style_colon): Description and declaration. * lib/quotearg.c (quotearg_n_style_colon): New function implementation.