MINOR: sample/acl: use is_idchar() to locate the fetch/conv name

MINOR: sample/acl: use is_idchar() to locate the fetch/conv name

Instead of scanning a string looking for an end of line, ')' or ',',
let's only accept characters which are actually valid identifier
characters. This will let the parser know that in %[src], only "src"
is the sample fetch name, not "src]". This was done both for samples
and ACLs since they are the same here.