FS-6186 --resolve

Thanks-to: Matteo Brancaleoni <mbrancaleoni@voismart.it>

FS-5544 after going over this with a fine tooth comb, all I could find was some issues behind asymetrical nat.  If there is a real problem here, post the flipping logs I asked for days ago.......

mod_v8: Added new extension class that makes it possible to subscribe to FS events. Wiki will be updated soon.

This commit changes behavior such that if --disable-core-odbc-support
is provided we'll build without ODBC even if the libraries are there.

Previously we would always quietly build with ODBC support if it was
on the system.

Contrary to what was said in commit 72a80498, my 2012 commit
ffc8e81b did not affect the behavior of --disable-core-odbc-support.
We never recognized the flag as being different from not providing the
option at all.

What the commit did do was to cause us to fail loudly if
--enable-core-odbc-support was provided but the system libraries were
not there.  This behavior is preserved.

(That commit also caused us to potentially run certain checks twice,
which this commit resolves.)

You can also now provide --enable-core-odbc-support=optional which has
the same effect as the default behavior.

FS-6173 --resolve

Thanks-to: James Le Cuirot <chewi@aura-online.co.uk>

This reverts commit dfd591e9, reversing
changes made to 998b6567.

This reverts commit ef35dc63.

...and rewrite entire block for better clarity of purpose.

We might want to look more closely at the AX_LIB_ODBC macro as well.

This amends commit dfd591e9.

system calls do not clear errno, just set it in case of failure, so we
gotta make sure the recv() syscall really failed by checking the return value

In commit ffc8e81b, tc ensured that
configure would abort if libodbc was not found. However this resulted
in the library check being done twice, as well as rendering
--disable-core-odbc-support ineffective. If libodbc was found, it
would enable core ODBC support regardless. This fix ensures the check
is only done once or not at all if core ODBC support is explicitly
disabled.
Signed-off-by: Travis Cross <tc@traviscross.com>