Grin: Use a working directory under the user home

[ignopeverell]

Currently grin uses the current directory to load different databases, write its logs, read configuration, save wallet files, etc. We should instead rely on a .grin under the user home like pretty much all other blockchain daemons.

[pcdinh]

Should it be configurable? I think that lots of hard core miners and users may want to move the working directory to a dedicated disk (mount point) for good reasons

[ignopeverell]

Yes of course, I should have been clearer that under the home folder should be a default, with a custom location configurable.

[haarts]

I think that using the XDG specification should be used here (that uses ~/.local and ~/.config). Most modern pieces of software do and that would be expected behavior.

[yeastplume]

Done via #1433