libquote.sh: Overhaul quote_command()
libquote.sh: Overhaul quote_command()

So far, quote_command() also used the simplicistic approach (see previous
commit) to quoting. Therefore, we adopt the more intelligent approach by
quoting arguments with quote_sh() instead of "rolling our own", making
bash(1) obsolete here, too.

While at it, quote_command() has been made 'set -u'-resilient, too.
Moreover, it reliably detects (as well as reports) usage errors and
failures (especially quote_sh() failures)!

donkarlos/libdl-sh