daemon: add add_sprintf

Bring the add_sprintf function for stringsbuf from the library.
daemon: move AUGEAS_ERROR to the common header

Other than for current aug_* API, it will be useful for further code
using augeas directly.
daemon: lvm-filter: use augeas for setting the filter

The way to set the filter for lvm devices was to open lvm.conf, look
for uncommented "filter =" lines and replace the configuration there.
This had the issue that if there is no uncommented filter line, then the
filter cannot be changed at all; considering newer lvm2 releases ship a
sample configuration with no uncommented filter lines, then the old way
became wrong and not sufficient.

Instead, take a copy of the upstream lvm.aug lens, with a simple change
to allow parsing also negative values in configuration, and install it
in the daemon. When asking to change the lvm filter, use augeas making
sure to use this custom lens for the lvm.conf copy used within the
appliance.