d/nfsconvert.py: add short "u" option for mountd's no-udp

Andreas Hasenackrequested to merge
ahasenack/nfs-utils:salsa-nfs-utils-nfsconvert-udp-option into master

The conversion script already handles rpc.mountd's --no-udp long option, but it's missing -u:

Setting up nfs-common (1:2.6.1-1ubuntu1~ppa1) ...
option -u not recognized
Error whilst converting RPCMOUNTDOPTS to nfsconf options.

Conversion failed. Please correct the error and try again.
dpkg: error processing package nfs-common (--configure):

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