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
This change does several things. First, and most important, it allows
dumping the "n" most recent stack frames on each debug() call. Second,
it demangles the C++ symbols. Third, it prepends each debug() message
with the debug level.
Unrelated to the above I've replaced all `assert(!is_forked_child());`
statements with `ASSERT_IS_NOT_FORKED_CHILD()` for consistency.
