BUG/MINOR: arg: Fix leaks during arguments validation for fetches/converters

BUG/MINOR: arg: Fix leaks during arguments validation for fetches/converters

Some sample fetches or sample converters uses a validation functions for their
arguments. In these function, string arguments (ARGT_STR) may be converted to
another type (for instance a regex, a variable or a integer). Because these
strings are allocated when the argument list is built, they must be freed after
a conversion. Most of time, it is done. But not always. This patch fixes these
minor memory leaks (only on few strings, during the configuration parsing).

This patch may be backported to all supported versions, most probably as far as
2.1 only. If this commit is backported, the previous one 73292e9e6 ("BUG/MINOR:
lua: Duplicate map name to load it when a new Map object is created") must also
be backported. Note that some validation functions does not exists on old
version. It should be easy to resolve conflicts.