The xtrace (set -x) output in dash is a bit of a pain, because arguments
containing whitespace aren't quoted. This can it impossible to tell
which bit belongs to which argument:

$ dash -c 'set -x; printf "%s\n" one "two three" four'
+ printf %s\n one two three four
one
two three
four

Another disadvantage is you cannot simply copy and paste the commands
from this xtrace output to repeat them for debugging purposes.

I wrote the attached patch which fixes this. I've been testing it for
about a week and had no issues.

The patch changes the following:

- Since we don't want every command name and argument quoted but only
those containing non-shell-safe characters, single_quote() has acquired
an extra argument indicating whether quoting should be conditional upon
the string containing non-shell-safe characters (1) or unconditional
(0). Shell-safe characters are defined as this set:
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz%+,./:=@_^!-

- Quoting of variable assignments preceding commands needs to be handled
specially; in order for the output to be suitable for re-entry into the
shell, only the part after the "=" must be quoted. For this purpose, I
changed eprintlist() into two functions, eprintvarlist() to print the
variable assignments and eprintarglist() to print the rest.

Hope this is useful,

- Martijn