string-util: rework strextend() to optionally inset separators between each appended string

This adds a new flavour of strextend(), called
strextend_with_separator(), which takes an optional separator string. If
specified, the separator is inserted between each appended string, as
well as before the first one, but only if the original string was
non-empty.

This new call is particularly useful when appending new options to mount
option strings and suchlike, which need to be comma-separated, and
initially start out from an empty string.