BUG/MINOR: cfgparse/proxy: Fix some leaks during proxy section parsing

BUG/MINOR: cfgparse/proxy: Fix some leaks during proxy section parsing

Allocated variables are now released when an error occurred during
use_backend, use-server, force/ignore-parsing, stick-table, stick and stats
directives parsing. For some of these directives, allocation errors have
been added.

This patch may be backported to all stable version but it only fixes leaks
or allocation errors during configuration parsing. Thus, it is not
mandatory. It should fix issue #1119.