add --password-on-stdin

[mafgh]

This allows the password to be read from stdin, e.g. from a password manager. Closes #1063.

[DimitriPapadopoulos]

Not sure if this PR is the place to address these issues, but:

1. What should happen if we pass --password-on-stdin multiple times? Right now openfortivpn attempts to read the password multiple times. I believe it should only try once.
2. When omitting the server or passing invalid arguments, I believe openfortivpn should exit before asking for the password.

$ openfortivpn --password-on-stdin --password-on-stdin
<password1>
<password2>
WARN:   Could not load configuration file "/volatile/local/opefortivpn/etc/openfortivpn/config" (No such file or directory).
ERROR:  Specify a valid host:port couple.
Usage: openfortivpn [<host>[:<port>]] [-u <user>] [-p <pass>] [--password-on-stdin]
[...]
$ 

The --cookie-on-stdin option has the same issues, so I am OK with fix both options in a different PR.

[mafgh]

How about we just set a flag during argument parsing:

if (strcmp(long_options[option_index].name, "password-on-stdin") == 0) {
  password_from_stdin = 1;
  break;
}

And then, just before this line:

openfortivpn/src/main.c

Line 673 in 912f1af

if (cli_cfg.password_set) {

read from stdin (if that flag is set), etc.

[DimitriPapadopoulos]

The flag will fix issue 1, but not issue 2. And there's an additional issue:
3. Option --password is not compatible with option --password-on-stdin, using both should raise an error.

Again, --cookie-on-stdin shares this issue. We don't have to fix everything here, I'll let you decide which issues you're willing to fix in this PR.

[mafgh]

I have tried to fix 1. and 3.

[rempairamore]

If you want to store passwords in the macOS Keychain, check out the GUI client I built on top of openfortivpn:
rempairamore/OpenFortiMac

Hope you find it useful!