issue-571: attemped to replace my_bool with c99 bool for mysql bindings

[junkerjoe]

This fix is in reference to #571

I'm not well versed in C, but decided to take a try at resolving this.

[fare]

LGTM. Did you test running it?

[vyzo]

LGTM overall. My only concern is what happens with old mysqlclient versions who still use my_bool, will it still work?
If not we might have to add some conditional expansion trickery.

[junkerjoe]

At this point I've tried compiling Gerbil and making a few basic queries through Gerbil's MySQL driver with the following environments:

• Ubuntu 20.04.1 with MySQL 8.0.21
• Ubuntu 16.04.7 with MySQL 5.7.31 (pre-my_bool patch)

Both seem to work, however I didn't make a lot of queries to test with. When compiling under MySQL 5.7.31, I get the following compiler warning that is relevant to the changes made:

[*] Building gerbil stdlib
... compile foreign db/_mysql
../../../src/std/db/_mysql.c: In function ‘ffi_mysql_bind_set_null’:
../../../src/std/db/_mysql.c:2036:20: warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]
  mybind[k].is_null = is_null;
                    ^

Other than that, things seem to work with a very basic level of testing done.