st: Minor bug in -labvalclone- by NJC

[Sergiy Radyakin <serjradyakin@gmail.com>]

Bug in -labvalclone- by NJC
http://fmwww.bc.edu/repec/bocode/l/labvalclone.ado

===Example:
clear
label define a 1 "US" 2 "UK" 3 "UA"
labvalclone a aa

===Output:
unrecognized command:  laabel
r(199);

===Why does it happen:
Because replacing blindly code is not safe. Nick is trying to check
for this problem by having a safety check of the old argument earlier
in the program:
if "`old'" == "label" | "`old'" == "define"
but later is doing a replacement of a SUBSTRING, not a WORD-by-word replacement.

===Suggested fix:
Simply flank both args (old and new) with spaces:
local line: subinstr local line " `old' " " `new' "

Best,
   Sergiy Radyakin
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