ovsdb-server: Maintain the database lock with --detach.

ovsdb-server: Maintain the database lock with --detach.

Before this commit, "ovsdb-server --detach" would detach after it opened
the database file, which meant that the child process did not hold the
file lock on the database file (because a forked child process does not
inherit its parents' locks).  This commit fixes the problem by making
ovsdb-server open the database only after it has detached.  This fix, in
turn, required that daemonize() not chdir to /, because this would break
databases whose names are given relative to the current directory, and so
this commit also changes ovsdb-server to do so later.