gensubst: Make template head and foot tunable
gensubst: Make template head and foot tunable

Template head and foot are tuned by specifying a 'template=TEMPLATE'
pair, where TEMPLATE is the (literal) head followed by any (abstract)
IDENTifier, followed by the (literal) foot; e.g. "template='${N}'"
recognizes sh(1) parameter substitutions like '${some_variable}' (but
no positional parameter substitutions like '${1}' because numbers
aren't IDENTifiers.

donkarlos/libdl-sh