libsystemd-network: make option_append() atomic and make the code a bit clearer

libsystemd-network: make option_append() atomic and make the code a bit clearer

Comparisons are done in the normal order (if (need > available), not if (available < need)),
variables have reduced scope and are renamed for clarity.

The only functional change is that if we return -ENAMETOOLONG, we do that
without modifying the options[] array.

I also added an explanatory comment. The use of one offset to point into three
buffers is not obvious.

Coverity (in CID#1402354) says that sname might be accessed at bad offset, but
I cannot see this happening. We check for available space before writing anything.