options: Complete the transition to numeric toggles (`#if')

For the sake of review, this commit alters only the code; the affiliated
comments within the source files also need to be updated, but doing so
now would obscure the operational changes that have been made here.

* All on/off options have been switched to the numeric `#if' variant;
  that is the only way to make this `default_options.h.in' thing work
  in a reasonable manner.

* There is now some very minor compile-time checking of the user's
  choice of options.

* NO_FAST_EXPTMOD doesn't seem to be used, so it has been removed.

* ENABLE_USER_ALGO_LIST was supposed to be renamed DROPBEAR_USER_ALGO_LIST,
  and this commit completes that work.

* DROPBEAR_FUZZ seems to be a relatively new, as-yet undocumented option,
  which was added by the following commit:

    commit 6e0b539e9c
    Author: Matt Johnston <matt@ucc.asn.au>
    Date:   Tue May 23 22:29:21 2017 +0800

        split out checkpubkey_line() separately

  It has now been added to `sysoptions.h' and defined as `0' by default.

* The configuration option `DROPBEAR_PASSWORD_ENV' is no longer listed in
  `default_options.h.in'; it is no longer meant to be set by the user, and
  is instead left to be defined in `sysoptions.h' (where it was already being
  defined) as merely the name of the environment variable in question:

    DROPBEAR_PASSWORD

  To enable or disable use of that environment variable, the user must now
  toggle `DROPBEAR_USE_DROPBEAR_PASSWORD'.

* The sFTP support is now toggled by setting `DROPBEAR_SFTPSERVER', and the
  path of the sFTP server program is set independently through the usual
  SFTPSERVER_PATH.

cli-auth.c

@@ -60,9 +60,11 @@ void cli_auth_getmethods() {
 	*/
 	if (ses.keys->trans.algo_comp != DROPBEAR_COMP_ZLIB_DELAY) {
 		ses.authstate.authtypes = AUTH_TYPE_PUBKEY;
+#if DROPBEAR_USE_DROPBEAR_PASSWORD
 		if (getenv(DROPBEAR_PASSWORD_ENV)) {
 			ses.authstate.authtypes |= AUTH_TYPE_PASSWORD | AUTH_TYPE_INTERACT;
 		}
+#endif
 		if (cli_auth_try() == DROPBEAR_SUCCESS) {
 			TRACE(("skipped initial none auth query"))
 			/* Note that there will be two auth responses in-flight */
@@ -335,7 +337,7 @@ char* getpass_or_cancel(const char* prompt)
 {
 	char* password = NULL;
 	
-#ifdef DROPBEAR_PASSWORD_ENV
+#if DROPBEAR_USE_DROPBEAR_PASSWORD
 	/* Password provided in an environment var */
 	password = getenv(DROPBEAR_PASSWORD_ENV);
 	if (password)