Allow for custom options to pg_dumpall and custom formats

[FinweVI]

Hello,

this PR allow for passing custom options to pg_dumpall by using the $POSTGRES_HOST_OPTS variable.
My use case is that I use a managed DB, and the default "admin" user need to connect on a different default DB than postgres or template1.
pg_dumpall support this with the -l option, but I can't pass it as an argument with the current code.

I also added a $POSTGRES_FORMAT variable, defaulting to plain (p), same as before.
I prefer to use -Fc or the custom PostGres format whenever I can, with pg_dump.

Please let me know if everything looks OK!