Use doas on host if sudo is not installed

My attempt at solving #2051 (closed)

I got it mostly working. See the TODO. Also, when I run pmbootstrap install, I get multiple doas prompts for my password which is really annoying:

[anjan@navi ~/code/pmbootstrap]$ ./pmbootstrap.py install
[23:16:09] *** (1/4) PREPARE NATIVE CHROOT ***
doas (anjan@navi) password:
doas (anjan@navi) password:
doas (anjan@navi) password:
doas (anjan@navi) password:
doas (anjan@navi) password:
doas (anjan@navi) password:

I dont think I will be able to hack on this anymore but my work so far is available for anyone else to build on. Cheers!

Closes #2051 (closed)