sysctl-util: use read_full_virtual_file() for reading sysctls

Given these files are part of procfs, let's use the correct API calls
for reading them.

This changes one occasion of read_one_line_file() to
read_full_virtual_file(), which superficially is a different thing, but
shouldn't actually be a difference, since sysctls can't be longer than
4K anyway, and the piecemeal logic behind read_one_line_file() cannot
work with the special semantics of procfs anyway.