quotearg: add support for $'' shell escaping * lib/quotearg.h: Add "shell-escape" and "shell-escape-always" items and descriptions. * lib/quotearg.c (quotearg_buffer_restyled): Add support for the above types by quoting like "shell", but using $'...' syntax for non printable characters, which should provide unambiguous printable output for any input. * tests/test-quotearg-simple.c: Update accordingly.