Calendar.Timezone.Runtime_timezone_compiler: Improve reentrancy.

The runtime timezone compiler was not thread safe, and could
fail with the compiler error "Undefined identifier forever."
when multiple concurrent threads compiled the same timezone.

Potentially fixes [bug 6816] #1:1.
release number bumped to 784 by export.pike
release number bumped to 785 by export.pike
Mysql: Support the MariaDB client library.

The MariaDB client library is a forward port of the LGPL
mysql client library from MySQL 3.23 to support modern
MySQL and MariaDB.
Mysql: Added option to disable use of MariaDB-client.
Tools.Standalone.features: Version and license for Mysql.

The feature listers now know about how to detect MariaDB.
release number bumped to 786 by export.pike
release number bumped to 787 by export.pike
Mysql: Add include directives first.

This should hopefully fix the "WARNING: Header version does not match
library version, diabling module" problem.
release number bumped to 788 by export.pike
release number bumped to 789 by export.pike
release number bumped to 790 by export.pike
release number bumped to 791 by export.pike
Mysql: libssl is needed by modern lib{mysql,mariadb}client.
release number bumped to 792 by export.pike
release number bumped to 793 by export.pike
Mysql: Added Mysql.client_info().
release number bumped to 794 by export.pike
release number bumped to 795 by export.pike
[compiler] Avoid recursion on the C-stack.

las.c:count_args() now uses the parent links (as per an old FIXME),
to avoid running out of C-stack when counting huge argument lists.

Fixes [bug 6860].