pgsql: Create a function variable "join_search_hook" to let plugins

Log Message:
-----------
Create a function variable "join_search_hook" to let plugins override the
join search order portion of the planner; this is specifically intended to
simplify developing a replacement for GEQO planning. Patch by Julius
Stroffek, editorialized on by me. I renamed make_one_rel_by_joins to
standard_join_search and make_rels_by_joins to join_search_one_level to better
reflect their place within this scheme.

Modified Files:
--------------
pgsql/src/backend/optimizer:
README (r1.39 -> r1.40)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/README?r1=1.39&r2=1.40)
pgsql/src/backend/optimizer/path:
allpaths.c (r1.164 -> r1.165)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/allpaths.c?r1=1.164&r2=1.165)
joinrels.c (r1.86 -> r1.87)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/joinrels.c?r1=1.86&r2=1.87)
pgsql/src/include/optimizer:
paths.h (r1.98 -> r1.99)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/paths.h?r1=1.98&r2=1.99)