library: stop treating lxcname differently, <PIDS> api

library: stop treating lxcname differently, <PIDS> api

Unlike other cached string fields (such as USER name),
that lxcname field was defined as 'const char *'. This
required a custom 'set' function using a cast in order
to avoid a compiler warning regarding that assignment.

So this commit brings lxcname into line with all those
other cached string fields. And while we're at it, the
reason some string fields have no 'freefunc' will also
be explained with some new comments in the Item_table.

Signed-off-by: Jim Warner <james.warner@comcast.net>