bug#16360: "guild help COMMAND" crashes

[Zefram]

"guild help COMMAND" crashes for most existing guild subcommands.
For example:

$ GUILE=/usr/bin/guile-2.0 guild help frisk 
Usage: guild frisk OPTION...
Show dependency information for a module.
Backtrace:
In ice-9/boot-9.scm:
 157: 8 [catch #t #<catch-closure 25bc500> ...]
In unknown file:
   ?: 7 [apply-smob/1 #<catch-closure 25bc500>]
In ice-9/boot-9.scm:
  63: 6 [call-with-prompt prompt0 ...]
In ice-9/eval.scm:
 432: 5 [eval # #]
In /usr/bin/guild:
  74: 4 [main ("/usr/bin/guild" "help" "frisk")]
In scripts/help.scm:
 181: 3 [main "frisk"]
 155: 2 [show-help #<directory (scripts frisk) 26d1900> #<output: file 
/dev/pts/14>]
In ice-9/boot-9.scm:
 788: 1 [call-with-input-file #f ...]
In unknown file:
   ?: 0 [open-file #f "r" #:encoding #f #:guess-encoding #f]

ERROR: In procedure open-file:
ERROR: Wrong type (expecting string): #f
$

This is guile-2.0.9 on Debian.  Debian incarnation of this bug report:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=734314

-zefram