On-demand imports for crypt and nis in account backends.
On-demand imports for crypt and nis in account backends.

Patch from Thilo-Alexander Ginkel. The crypt and nis modules are not available
under Windows, but were imported unconditionally, so that using custom backends
under Windows failed. This patch should fix this by performing the imports
only when needed.

Fixes bug 1244.
Reviewed at http://reviews.review-board.org/r/976/




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