command.c: Replace usleep() by nanosleep().

POSIX.1-2001 declares usleep() obsolete and POSIX.1-2008 removes the
specification. Fortunately, there is only a single user of usleep()
in the tree: com_sender() which needs to retry the command if another
sender command is currently running. This commit changes com_sender()
to call nanosleep() instead of usleep() and adds a comment which
explains why we are going to sleep at this point.