fix(mounts): escape special characters in fstab mount paths (#6911)

Mount points containing spaces (and tabs/newlines/backslashes) are now
octal-escaped (\040, \011, \012, \134) when written to /etc/fstab, so
`mount -a` can parse them. Directories are still created with real
characters.

The escape/unescape helpers live in `cloudinit.util` and are reused by
`util.mounts()`, which previously only undid `\040` for spaces and now
decodes tabs, newlines and backslashes as well.

Add unit tests plus a one-off integration test (`test_gh3603.py`) and
document the behaviour in the `cc_mounts` examples.

Fixes GH-3603