install: support password as cmdline arg

Add a --password option to pmbootstrap install, should simplify BPO and finally make the process fully non-interactive. There are some upcoming use cases for this too (such as https://git.sr.ht/~martijnbraam/rptr).

Based on the original implementation in !1919 (closed)

I added a BIG note to the help output, explaining that any password you enter is very much NOT a secure password.

Closes: #1908 (closed)