formpost: support quotes, commas and semicolon in file names

- document the double-quote and backslash need be escaped if quoting.
- libcurl formdata escape double-quote in filename by backslash.
- curl formparse can parse filename both contains '"' and ',' or ';'.
- curl now can uploading file with ',' or ';' in filename.

Bug: http://curl.haxx.se/bug/view.cgi?id=1171

tests/data/test39

@@ -26,7 +26,7 @@ http
 HTTP RFC1867-type formposting with filename= and type=
  </name>
  <command>
-http://%HOSTIP:%HTTPPORT/we/want/39 -F name=daniel -F tool=curl --form-string "str1=@literal" --form-string "str2=<verbatim;type=xxx/yyy" -F "file=@log/test39.txt;type=moo/foobar;filename=fakerfile" -F file2=@log/test39.txt
+http://%HOSTIP:%HTTPPORT/we/want/39 -F name=daniel -F tool=curl --form-string "str1=@literal" --form-string "str2=<verbatim;type=xxx/yyy" -F "file=@log/test39.txt;type=moo/foobar;filename=fakerfile" -F file2=@log/test39.txt -F "file3=@\"log/test39.txt\";type=mo/foo;filename=\"f\\\\\\\\ak\\\\\\er,\\\\an\\d;.t\\\"xt\"" -F 'file4=@"log/test39.txt"; filename="A\\AA\"\"\\\"ZZZ"'
 </command>
 # We create this file before the command is invoked!
 <file name="log/test39.txt">
@@ -47,7 +47,7 @@ POST /we/want/39 HTTP/1.1
 User-Agent: curl/7.10.4 (i686-pc-linux-gnu) libcurl/7.10.4 OpenSSL/0.9.7a ipv6 zlib/1.1.3
 Host: %HOSTIP:%HTTPPORT
 Accept: */*
-Content-Length: 810
+Content-Length: 1184
 Expect: 100-continue
 Content-Type: multipart/form-data; boundary=----------------------------24e78000bd32
 
@@ -85,6 +85,24 @@ This is a bar foo
 bar
 foo
 
+------------------------------24e78000bd32
+Content-Disposition: form-data; name="file3"; filename="f\\\\ak\\\\er,\\an\\d;.t\"xt"
+Content-Type: mo/foo
+
+foo bar
+This is a bar foo
+bar
+foo
+
+------------------------------24e78000bd32
+Content-Disposition: form-data; name="file4"; filename="A\\AA\"\"\\\"ZZZ"
+Content-Type: text/plain
+
+foo bar
+This is a bar foo
+bar
+foo
+
 ------------------------------24e78000bd32--
 </protocol>
 </verify>