treewide: replace unsafe string functions

treewide: replace unsafe string functions

Replace sprintf(), strncpy() etc. with safer variants that perform bounds
checking on the target buffer. Also rework unsafe `p += sprintf(p, ....)`
code to properly handle error cases.

Ref: http://lists.infradead.org/pipermail/openwrt-devel/2020-May/023363.html
Suggested-by: Philip Prindeville <philipp@redfish-solutions.com>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>