bug#24335: Error when invoking usermod while reconfiguring

[Leo Famulari <leo@famulari.name>]

While reconfiguring my GuixSD system from 1062f2451 (gnu: Add stagit),
the process fails as show below.

When I revert 2d1a2f4af (system: Add 'create-home-directory?' field to
<user-account>) and the associated commit cb58b2cd, the reconfigure
completes successfully.

[...]
activating system...
setting up setuid programs in '/run/setuid-programs'...
populating /etc from /gnu/store/xsmb4mfnm2m10zmdiszsdah7wjwvfb5z-etc...
usermod: no changes
Backtrace:
In guix/ui.scm:
1190: 19 [run-guix-command system "reconfigure" ...]
In ice-9/boot-9.scm:
 157: 18 [catch srfi-34 #<procedure 659b460 at guix/ui.scm:423:2 ()> ...]
 157: 17 [catch system-error #<procedure 4fa6f60 at ice-9/eval.scm:416:20 ()> ...]
In ice-9/eval.scm:
 481: 16 [lp (#<fluid 32>) (#t)]
In ice-9/r4rs.scm:
  90: 15 [dynamic-wind #<procedure 5391be0 at ice-9/boot-9.scm:1133:2 _> ...]
In guix/store.scm:
1182: 14 [run-with-store # #<procedure 2d34f90 at ice-9/eval.scm:416:20 (a)> # ...]
In ice-9/r4rs.scm:
  39: 13 [call-with-values #<procedure 671a960 at ice-9/eval.scm:416:20 ()> ...]
In ice-9/eval.scm:
 386: 12 [eval #<memoized ((<3> <1>) <0>)> (# # # ...)]
 432: 11 [eval # #]
In ice-9/r4rs.scm:
  90: 10 [dynamic-wind #<procedure adea900 at ice-9/boot-9.scm:1133:2 _> ...]
  90: 9 [dynamic-wind #<procedure adea800 at ice-9/boot-9.scm:1133:2 _> ...]
In unknown file:
   ?: 8 [primitive-load "/gnu/store/ikbva4qvlgxppdy522qn6graq05g3b8p-activate"]
In ice-9/eval.scm:
 432: 7 [eval # ()]
In ice-9/boot-9.scm:
 768: 6 [for-each #<procedure primitive-load (_)> #]
In unknown file:
   ?: 5 [primitive-load "/gnu/store/rx8dd8z03xcf252c9xrzz9dipbliql6a-activate-service"]
In ./gnu/build/activation.scm:
 252: 4 [activate-users+groups (# # # # ...) (# # # # ...)]
In srfi/srfi-1.scm:
 616: 3 [for-each #<procedure activate-user (expr)> (# # # # ...)]
In ./gnu/build/activation.scm:
 217: 2 [activate-user ("guixbuilder01" 30001 "guixbuild" ...)]
 177: 1 [modify-user "guixbuilder01" "guixbuild" ...]
In unknown file:
   ?: 0 [system* "usermod" "-u" ...]

ERROR: In procedure system*:
ERROR: Wrong type (expecting string): #t