mount-tool: use libmount to parse /proc/self/mountinfo

mount-tool: use libmount to parse /proc/self/mountinfo

Same motivation as in other places: let's use a single logic to parse this.

Use path_equal() to compare the path.

A bug in error handling is fixed: if we failed after the GREEDY_REALLOC but
before the line that sets the last item to NULL, we would jump to
_cleanup_strv_free_ with the strv unterminated. Let's use GREEDY_REALLOC0
to avoid the issue.