[VAR] Fix varinit ordering that broke fc

[VAR] Fix varinit ordering that broke fc

Git commit 0df96793ef6aa103df228d7dfe56099b7d721a15 "[SHELL] Add
preliminary LINENO support" added the LINENO variable in the middle of
other initialized variables, causing some macros for TERM and HISTSIZE
to break (both of these are only used if libedit support is compiled in,
which is not the case by default).

The breakage is the same as can be seen by setting HISTSIZE=0.

Also add a comment warning about this.

Reported-by: Wez Furlong <kingwez@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>