proxies for FTP urls.

fixed some AC_SUBST configure entries.

Removed and fixed some AC_SUBST configure entries.

to the specific tracker entries

it. This caused a segfault in some fprintf() implementations. Like on Solaris.

- Bug report #1792649 (http://curl.haxx.se/bug/view.cgi?id=1792649) pointed
  out a problem with doing an empty upload over FTP on a re-used connection.
  I added test case 541 to reproduce it and to verify the fix.

- I noticed while writing test 541 that the FTP code wrongly did a CWD on the
  second transfer as it didn't store and remember the "" path from the
  previous transfer so it would instead CWD to the entry path as stored. This
  worked, but did a superfluous command. Thus, test case 541 now also verifies
  this fix.

  second transfer as it didn't store and remember the "" path from the
  previous transfer so it would instead CWD to the entry path as stored. This
  worked, but did a superfluous command. Thus, test case 541 now also verifies
  this fix.

  out a problem with doing an empty upload over FTP on a re-used connection.
  I added test case 541 to reproduce it and to verify the fix.

not success.

(to validate bug #1790403).

sockets.

underlying ftp_readresp() function has a separate "cache" where there might
in fact be leftover data...

sockets.

Renamed the curl_ftpssl enum to curl_usessl and its enumerated constants,
creating macros for backward compatibility.

generic, and more consistent with each other.

case 615 to test an SFTP upload failure.