mainUtils: match readline behavior when ABC_USE_READLINE is undefined

[maliberty]

The non-readline branch of Abc_UtilsGetUsersInput has three behavioral gaps
versus the readline branch that break callers driving abc as a coprocess
over a pipe — notably yosys's passes/techmap/abc.cc, which spawns abc -s
with piped stdin/stdout and uses read_until_abc_done to wait for
abc NN> <command> lines:

1. Missing flush on the prompt. The prompt is written with fprintf()
   and never flushed. On a pipe stdout is fully buffered, so the prompt
   never reaches the reader. The reader waits for the prompt, abc waits
   in fgets(), deadlock.

2. No echo of input. readline() emits each character to its output
   stream; yosys's protocol depends on seeing abc NN> source ...\n in
   the output to advance state. Without an echo it waits forever.

3. EOF ignored. When stdin closes, fgets() returns NULL but the
   function returns a stale Prompt buffer, causing a tight loop on pipe
   close. The readline branch exit(0)s on NULL.

This patch fixes all three. The echo is gated on !isatty(fileno(stdin))
because on a tty the kernel already echoes during cooked input, and a
double echo would be visible to interactive users.

Reproduction

Without the patch, with an ABC_USE_READLINE=0 build:

$ printf 'version\nquit\n' | ./abc -s
UC Berkeley, ABC 1.01 ...
abc 01> UC Berkeley, ABC 1.01 ...

The prompt appears (only because the pipe drained on exit, not on
flush) and the input is never echoed. yosys, which expects to see
`abc 01> source <script>` in the stream, hangs in `read()`.

With the patch, the same invocation produces:

$ printf 'version\nquit\n' | ./abc -s
UC Berkeley, ABC 1.01 ...
abc 01> version
UC Berkeley, ABC 1.01 ...
abc 02> quit

which matches the output stream readline produces on the same input, and
unblocks yosys's coprocess driver.