Stop using deprecated getpass()

Stop using deprecated getpass()

The getpass() function is present in SUSv2, but marked LEGACY. It is
removed in POSIX.1-2001. Additionally, on Solaris getpass() returns
maximum 9 bytes. This will make longer passwords fail with no
possibility for user to know what is happening.

This patch removes usage of getpass() completely and replaces it with
direct implementation of what getpass() does: opens tty (existing code),
outputs the prompt (existing code), turns off echoing (new code), reads
one line (existing code shared with echoed mode), restores tty state
(new code) and closes tty (existing code).

Patch v2: incorporate review feedback, incl. style fixes, merge
          termios.h check in configure.ac with an existing
          AC_CHECK_HEADERS, add error check and logging after
          tcsettattr() when restoring tty settings

Signed-off-by: Tõivo Leedjärv <toivol@gmail.com>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20210328171151.12056-1-toivol@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21889.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>