BUG/MEDIUM: vars: make functions vars_get_by_{name,desc} thread-safe

BUG/MEDIUM: vars: make functions vars_get_by_{name,desc} thread-safe

This patch adds a lock to functions vars_get_by_name() and
vars_get_by_desc() to protect accesses to the list of variables.

After the variable is fetched, a sample data is duplicated by using
smp_dup() because the variable may be modified by another thread.

This should be backported to all versions supporting vars along with
"BUG/MINOR: sample: secure convs that accept base64 string and var name
as args" which this patch depends on.