Pike.gc_parameters(): Added some gc callbacks.

Adds callbacks that are called from the gc to simplify
debugging of memory leaks and similar.
release number bumped to 782 by export.pike
release number bumped to 783 by export.pike
Runtime: Improve robustness against outputting wide strings.

master()->handle_error() now survives if eg an object has
an _sprintf() that returns a wide string.

master()->compile_{error,warning}() now survive messages about
wide symbols.

Made a similar fix in Hilfe to survive compiler warnings and
errors about wide symbols.

Fixes [bug 6805].
SNMP: Fixed error in GetRequest variable bindings.

The variable value should be ASN1 "Null" rather than a bogus integer.
C.f. RFC 1905 section #3.
Stdio: Reduce number of system calls in mkdirhier().

This reduces the number of stats to the number of missing
path segments.
ADT.Struct: working int32 / SWord
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