Avoid interleaved $(info ) output

Avoid interleaved $(info ) output

Since $(info ) function output is written using two system calls for
the message and the newline, it's possible for output from another
parallel make job to sneak in between them.

Reported by Paul Eggert <eggert@cs.ucla.edu>, who saw the report from
Lars Ingebrigtsen <https://bugs.gnu.org/53358>.

* src/function.c (func_error): Paul provided a fix but instead I
rewrote the entire function: it's not possible for it to be invoked
with anything other than exactly one argument so don't worry about
re-combining the arguments.