func_odbc: Fix fixed size buffers fix (r414968).

The change that removed the fixed size buffers in odbc-related code --
removing arbitrary column width limits -- was incomplete. This change
adds: no segfault on writesql without insertsql and return value checks
after strdup.

While I was in the vicinity I cleaned up the linefeeds in the odbc
function descriptions, moved some code for clarity, removed some blobs
and noted (but didn't fix) that the 'odbc write ... exec' CLI command
doesn't behave as the dialplan equivalent when insertsql= is used.

ASTERISK-23582 #close
Review: https://reviewboard.asterisk.org/r/3579/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@414997 f38db490-d61c-443f-a65b-d21fe96a405b