core: Add optional unix socket file permissions (#4741)

* core: Add optional unix socket file permissions

This commit also changes the default unix socket file permissions to `u=w,g=,o=` (octal: `0200`).
It used to default to the shell's umask (usually `u=rwx,g=rx,o=rx`, octal: `0755`).

`/run/caddy.sock` -> `/run/caddy.sock` with `0200` default perms
`/run/caddy.sock|0222` -> `/run/caddy.sock` with `0222` perms

`|` instead of `:` is used as a separator, to account for the `:` in Windows drive letters (e.g. `C:\absolute\path.sock`)

Fun fact:
The old unix(7) man page (pre Jun 2016) stated a socket needs both read and write perms.
Turns out, only write perms are needed.
Corrected in https://github.com/mkerrisk/man-pages/commit/7578ea2f85b272363d22680d69e7d32f0b59c83b
Despite this, most implementations still default to read+write to this date.

* Add cases with Windows paths to test

* Require write perms for the owning user

0 files changed, 0 insertions, 0 deletions