--request-target: instead of --strip-path-slash

... and CURLOPT_REQUEST_TARGET instead of CURLOPT_STRIP_PATH_SLASH.

This option instead provides the full "alternative" target to use in the
request, instead of extracting the path from the URL.

Test 1298 and 1299 updated accordingly.

Idea-by: Evert Pot
Suggestion: https://daniel.haxx.se/blog/2017/06/19/options-with-curl/comment-page-1/#comment-18373

Closes #1593

src/tool_help.c

@@ -160,8 +160,8 @@ static const struct helptxt helptext[] = {
    "Disable URL sequences and ranges using {} and []"},
   {"-I, --head",
    "Show document info only"},
-  {"-H, --header <header>",
-   "Pass custom header LINE to server"},
+  {"-H, --header <header/@file>",
+   "Pass custom header(s) to server"},
   {"-h, --help",
    "This help text"},
   {"    --hostpubmd5 <md5>",
@@ -298,8 +298,8 @@ static const struct helptxt helptext[] = {
    "Set a CRL list for proxy"},
   {"    --proxy-digest",
    "Use Digest authentication on the proxy"},
-  {"    --proxy-header <header>",
-   "Pass custom header LINE to proxy"},
+  {"    --proxy-header <header/@file>",
+   "Pass custom header(s) to proxy"},
   {"    --proxy-insecure",
    "Do HTTPS proxy connections without verifying the proxy"},
   {"    --proxy-key <key>",
@@ -352,6 +352,8 @@ static const struct helptxt helptext[] = {
    "Set the remote file's time on the local output"},
   {"-X, --request <command>",
    "Specify request command to use"},
+  {"    --request-target",
+   "Specify the target for this request"},
   {"    --resolve <host:port:address>",
    "Resolve the host+port to this address"},
   {"    --retry <num>",
@@ -400,8 +402,6 @@ static const struct helptxt helptext[] = {
    "Use SSLv3"},
   {"    --stderr",
    "Where to redirect stderr"},
-  {"    --strip-path-slash",
-   "Strip off the first slash of the path"},
   {"    --suppress-connect-headers",
    "Suppress proxy CONNECT response headers"},
   {"    --tcp-fastopen",