abuild.conf: define format-security and int-conversion errors

format-security warns of usage such as `printf(x)`, which is usually a
security hole.

int-conversion is very useful to find cases such as

error: assignment to 'const char *' from 'int' makes pointer from integer without a cast [-Werror=int-conversion]
 msg = strerror_r(errnum, buf, buflen);

where the usage of things like the wrong strerror_r are legitimate
errors in the application that cause it to crash. it makes more sense
for the compiler to reject it instead, and this does that.
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