If the channel is locked when calling ast_set_variables and any of the
variables contained dialplan functions, there's a possiblilty of a deadlock.
To prevent this, either the explicit locks were removed or the call to
ast_set_variables moved out of the lock scope. A warning to not hold
channel locks is also added to the documentation for ast_set_variables.
Resolves: #1936
3f0ddb0f3a