BUG/MINOR: lua/vars: prevent get_var() from allocating a new name

BUG/MINOR: lua/vars: prevent get_var() from allocating a new name

Variable names are stored into a unified list that helps compare them
just based on a pointer instead of duplicating their name with every
variable. This is convenient for those declared in the configuration
but this started to cause issues with Lua when random names would be
created upon each access, eating lots of memory and CPU for lookups,
hence the work in 2.2 with commit 4e172c93f ("MEDIUM: lua: Add
`ifexist` parameter to `set_var`") to address this.

But there remains a corner case with get_var(), which also allocates
a new variables. After a bit of thinking and discussion, it never
makes sense to allocate a new variable name on get_var():
  - if the name exists, it will be returned ;
  - if it does not exist, then the only way for it to appear will
    be that some code calls set_var() on it
  - a call to get_var() after a careful set_var(ifexist) ruins the
    effort on set_var().

For this reason, this patch addresses this issue by making sure that
get_var() will never cause a variable to be allocated. This is done
by modifying vars_get_by_name() to always call register_name() with
alloc=0, since vars_get_by_name() is exclusively used by Lua and the
new CLI's "get/set var" which also benefit from this protection.

It probably makes sense to backport this as far as 2.2 after some
observation period and feedback from users.

For more context and discussions about the issues this was causing,
see https://www.mail-archive.com/haproxy@formilux.org/msg40451.html
and in issue #664.

(cherry picked from commit 89f6dedf486823237ffbe02f8d78df454332c615)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>