ovs-thread: New function ovsthread_id_self().

ovs-thread: New function ovsthread_id_self().

I foresee a need for possibly large numbers of instances of "struct
seq" (which is introduced in an upcoming patch).  Each struct seq
needs some per-thread data.  POSIX has pthread_key_t for this, but
the number of keys can be fairly limited, to as few as 128.  It is
reasonable to work around this by using a hash table indexed on the
current thread.  That only works if one can get a thread identifier
that is hashable (pthread_t is not).  This patch introduces a
hashable thread identifier.

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