db: improve mapping of input-keys to DB columns

db: improve mapping of input-keys to DB columns

Currently, we copy the key-name to a buffer, iterate over it to replace
the full-stops with underscores, using `strchr` from the start of the
buffer on each iteration, then append the buffer to the SQL statement.

Apart from the inefficiency, `strncpy` is used to do the copies, which
leads gcc to complain:

  ../../util/db.c:118:25: warning: `strncpy` output may be truncated copying 31 bytes from a string of length 31

Furthermore, the buffer is one character too short and so there is the
possibility of overruns.

Instead, append the key-name directly to the statement using `sprintf`,
and run `strchr` from the last underscore on each iteration.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>