fs: allow backslashes in paths via buckconfig

[daandemeyer]

File names and relative paths reject backslashes on every platform. On
Windows a backslash is a path separator, but on other platforms it is a
valid path character that occurs in practice. Building OS images, for
example, can produce systemd-escaped names such as
system-systemd\x2dfoo.slice, which Buck2 currently rejects.

Keep rejecting backslashes by default, but add
[buck2] allow_backslashes_in_paths = true to permit them on platforms
where they are not path separators. Treat the setting as daemon startup
configuration so changing it restarts buckd and one daemon cannot mix
path invariants. Windows continues to reject literal backslashes.

Initialize the low-level path policy from the startup configuration in
both the client and daemon. Same-value initialization is idempotent for
--no-buckd, while conflicting values are rejected. This also preserves
the original concrete error type of from_system_path, since validation
no longer performs a fallible environment lookup.

Cover policy initialization, platform behavior, and all affected path
validators in buck2_fs tests. Cover buckconfig parsing in buck2_common,
and document the opt-in in the buckconfig reference.

Signed-off-by: Daan De Meyer daan@amutable.com

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D110922815. (Because this pull request was imported automatically, there will not be any future comments.)

[lf-]

I feel vaguely like this should be a buckconfig setting instead; the ergonomics of passing in an env-var are not ideal.

[daandemeyer]

@lf- Reworked to a buck config option