Update M_PREFIX behaviour to default to a writeable directory

[stennie]

To simplify installation, the M_PREFIX environment variable should default to the user's home directory instead of /usr/local/. The /usr/local default originates from npm install -g.

Expected behaviour variation for npm installation:

• if M_PREFIX is not set, infer default via basepath for m (see: https://docs.npmjs.com/files/folders#tldr)

Expected behaviour for install via wget:

• if M_PREFIX is not set but /usr/local/m exists and is writeable, use /usr/local/m
• if M_PREFIX is not set but /usr/local/m exists and is not writeable, exit with message recommending sudo chown -R $(whoami) /usr/local/m
• if M_PREFIX is not set and /usr/local/m doesn't exist, use ~/m

General behaviour:

• if M_PREFIX is set, use this base path
• if M_PREFIX is set but the path isn't writeable, exit with message recommending sudo chown -R $(whoami) $M_PREFIX

[p-mongo]

~/m is rather dangerous, I frequently use single and double letter directory names for things I work with often. ~/.m is a safer choice, there is also ~/.local that e.g. python uses hence ~/.local/m is perhaps an even better choice.

I would also expect non-mutating operations like m and m ls to work fine when the user invoking them does not have write access to /usr/local/m (but if the latter exists, I expect m to list versions installed therein).

[stennie]

@p-mongo Apparently .local is part of the XDG spec so that seems a reasonable default. Thanks for the suggestion!

Agree on behaviour for non-mutating operations: only need to exit with an error if write access is required and the destination path is non-writeable.

Regards, Stennie