ovsdb: Allow ovsdb_log_open()'s caller to choose whether to lock.

ovsdb: Allow ovsdb_log_open()'s caller to choose whether to lock.

The current callers of ovsdb_log_open() always want to lock the file if
they are accessing it for read/write access.  An upcoming commit will add
a new caller that does not fit this model (it wants to lock the file
across a wider region) and so the caller should be able to choose whether
to do locking.  This commit adds that ability.

Also, get rid of the use of <fcntl.h> flags to choose the open mode, which
has always seemed somewhat crude and which this change would make even
cruder.