This teaches the status command to work with features.
'status features' will show a table listing all known features and whether
they are currently on or off.
`status test-feature` will test an individual feature, setting the exit status to
0 if the feature is on, 1 if off, 2 if unknown.
The `status` command currently silently allows incompatible flags (i.e.,
subcommands). Too, using flags to specify subcommands misleads the user
into thinking they can specify multiple subcommands.
We recently modified the `history` command to deprecate using flags for
subcommands. This change does the same for the `status` command.
Fixes#3509
Instead of globally marking the state as "in block" when evaluating
blocks/functions, update the "in block" status when pushing/popping
blocks on the parser stack.
Fixes#1729.
On a side note, `status -b` is actually pretty useless, because it
always returns 0 inside of a function (even without this patch).
