treewide: fix multiple compiler warnings

treewide: fix multiple compiler warnings

Fixes bunch of clang-15/gcc-10 compiler warnings, mostly related to
blobmsg_for_each_attr() usage:

 error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
 error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
 error: format string is not a string literal [-Werror,-Wformat-nonliteral]

Signed-off-by: Petr Štetiar <ynezz@true.cz>