Fix pg_amcheck --skip option parameter handling

Fix pg_amcheck --skip option parameter handling

The skip options set for all-visible and all-frozen were incorrect
as they used space rather than hyphen, causing a syntax error when
invoked. Also, the option for not skipping any pages at all, none,
was documented but not implemented.

Backpatch through 14 where pg_amcheck was introduced.

Bug: #17149
Reported-by: Chen Jiaoqian 
Reviewed-by: Masahiko Sawada 
Discussion: https://postgr.es/m/17149-5918ea748da36b15@postgresql.org
Backpatch-through: 14