ovs-thread: Add checking for mutex and rwlock initialization.

ovs-thread: Add checking for mutex and rwlock initialization.

With glibc, a mutex or rwlock filled with all-zero-bytes is properly
initialized for use, but this is not true for any other libc that OVS
supports.  However, OVS gets a lot more testing with glibc than any other
libc.  This means that developers keep introducing bugs that do not
manifest on the main development platform.

This commit should help avoid the problem, by reusing the existing 'where'
members to indicate whether a mutex or rwlock has been initialized.

Signed-off-by: Ben Pfaff <blp@nicira.com>