pgsql: Code review for GUC revert-values-if-removed-from-postgresql.conf

Log Message:
-----------
Code review for GUC revert-values-if-removed-from-postgresql.conf patch;
and in passing, fix some bogosities dating from the custom_variable_classes
patch.  Fix guc-file.l to correctly check changes in custom_variable_classes
that are attempted concurrently with additions/removals of custom variables,
and don't allow the new setting to be applied in advance of checking it.
Clean up messy and undocumented situation for string variables with NULL
boot_val.  Fix DefineCustomVariable functions to initialize boot_val
correctly.  Prevent find_option from inserting bogus placeholders for custom
variables that are simply inquired about rather than being set.

Modified Files:
--------------
    pgsql/src/backend/utils/cache:
        ts_cache.c (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/ts_cache.c?r1=1.2&r2=1.3)
    pgsql/src/backend/utils/misc:
        README (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/README?r1=1.5&r2=1.6)
        guc-file.l (r1.50 -> r1.51)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc-file.l?r1=1.50&r2=1.51)
        guc.c (r1.416 -> r1.417)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.416&r2=1.417)
    pgsql/src/include/utils:
        guc_tables.h (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc_tables.h?r1=1.33&r2=1.34)