settings: Only call MkdirAll on save

Previously settingsFileName() created the ~/.config/pprof directory
if it did not exist. However, this is not possible for user nobody,
since its home directory is set to "/nonexistent". Instead, only
create the directory when we actually attempt to save the file, so
the error will happen at the appropriate interaction. This als
prevents pprof from creating empty settings directories.

Fixes the following error when running pprof's web UI as user nobody:

    mkdir /nonexistent: permission denied

Add support for saving the current configuration settings as a named
configuration and reloading such a configuration later.

All of the configurations are stored in a settings file, by default:
~/.config/pprof.json

A struct named config replaces the old map[string]string which used
to store the set of configurable options. Instead, each option is
now a field in the config struct.

The UI now has a new 'Config' menu with

Save as: prompts for a name and saves current config.
Default: applies default config.
$X: for every config named X: applies settings from X.

The currently selected config is highlighted.

Every named config has a delete button to control deletion.

Some semi-related changes:
1. Both filefunctions and functions in the granularity group had an
   initial value of true, which is incorrect since these are mutually
   incompatible choices. Set filefunctions' initial value to false.
2. Renamed the group for sorting from "cumulative" to "sort".
3. Store testing.T in TestUI instead of leaving the field nil.