* lib/quotearg.c (quotearg_buffer_restyled): Switch to a read-only
scan of the string when we initially encounter a single quote when
shell quoting, so that if we then switch to a more concise quoting method
we will not have written beyond that returned length.
This is significant for sh-quote, which has separate routines
to determine the length and do the actual quoting.
* tests/test-quotearg.h: Reinstate the buffer bounds checking
now that we never write more than the returned length.