Stop rs_getpass() from spinning forever on a closed stdin

[jolavillette]

getch() forwards getchar(), whose EOF is -1. rs_getpass() stored it in an unsigned char, where it becomes 0xFF and never equals RETURN, so a closed stdin makes the read loop append one 0xFF byte per iteration for good.

Measured on retroshare-service built from current master, run as -U <sslid> < /dev/null:

• 23 MB of output in 90 seconds, one core at 100%
• the process does not answer SIGTERM — the spin never returns to the shutdown check, so only SIGKILL ends it

That is what a systemd unit or a container gets today whenever the passphrase is asked with no terminal attached. With the fix the same run exits in under a second with a 2.5 kB log.

Changes:

• ch becomes an int, EOF ends the read with an empty result.
• The Windows branch had the milder form of the same bug: a closed console returns EOF from every _getch() and the loop could only leave through the PASS_MAX branch, returning 512 bytes of 0xFF as if they had been typed. It now stops on EOF too, and accepts \n next to \r so a redirected stdin terminates the line.
• Optional eof out parameter, defaulting to nullptr so existing calls are unchanged. An empty string cannot distinguish "nothing typed" from "stream closed", and without that a caller looping "passphrase cannot be empty, try again" spins just as hard one level up.
• tcgetattr() return value checked: it fails whenever stdin is not a terminal, and t_old was then applied to stdin uninitialized.

Built and run on Linux (CMake+Ninja, Qt5). The only caller is retroshare-service; RetroShare/RetroShare#3290 makes that path much easier to reach, and a companion PR there bounds the prompt loops on top of this.