Simplify the writing of amoptions routines by introducing a convenience

Simplify the writing of amoptions routines by introducing a convenience
fillRelOptions routine that stores the parsed values in the struct using a
table-based approach.  Per Tom suggestion.  Also remove the "continue"
in HANDLE_*_RELOPTION macros, which were useless and in spirit they were
assuming too much of how the macros were going to be used.  (Note that these
macros are now unused, but the intention is to introduce some usage in a
future autovacuum patch, which is why they weren't completely removed.)

Also, do not call the string validation routine when not validating.  It seems
less error-prone this way, per commentary on the amoptions SGML docs.