Fix handling of mounts with spaces. This was surprisingly non-trivial.

Two issues compounded here: /proc/mounts escapes spaces with \040, and Dash
incorrectly processes escapes in quoted variables (echo "`echo "\\\\"`" outputs
a single slash).

In some places the issue was avoided via usage of mountpoint. In others, I had
to ensure that where backslashes are used, they are only used in single layer of
parsing to avoid incompatible double-processing.

That having been said, if the mountpoint contains grep-interpreted symbols or
backslashes, it will probably not work.