Replace a home-grown printf() clone with a fwopen() wrapper around

Replace a home-grown printf() clone with a fwopen() wrapper around
libc's vfprintf() that writes to a `struct output' instead of a file.

Print empty quotes ('') when an empty string is passed to outqstr().
This makes a difference for the trap builtin, where after "trap '' 0" we
printed "trap -- quit".  This is wrong, because an empty action means to reset
the action to the default.  A side effect of this commit is that empty
variables are now printed as "variable=''" instead of just "variable=".

Obtained-from:  FreeBSD