curl_easy_reset: release mime-related data.

Move curl_mime_initpart() and curl_mime_cleanpart() calls to lower-level
functions dealing with UserDefined structure contents.
This avoids memory leakages on curl-generated part mime headers.
New test 2073 checks this using the cli tool --next option: it
triggers a valgrind error if bug is present.

Bug: https://curl.haxx.se/mail/lib-2017-12/0060.html
Reported-by: Martin Galvan

tests/data/Makefile.inc

@@ -189,4 +189,4 @@ test2048 test2049 test2050 test2051 test2052 test2053 test2054 test2055 \
 test2056 test2057 test2058 test2059 test2060 test2061 test2062 test2063 \
 test2064 test2065 test2066 test2067 test2068 test2069 \
 \
-test2070 test2071 test2072
+test2070 test2071 test2072 test2073

tests/data/test2073

@@ -0,0 +1,68 @@
+<testcase>
+<info>
+<keywords>
+HTTP
+HTTP FORMPOST
+</keywords>
+</info>
+#
+# Server-side
+<reply>
+<data>
+HTTP/1.1 200 OK
+Date: Thu, 09 Nov 2010 14:49:00 GMT
+Content-Length: 10
+
+contents1
+</data>
+<data1>
+HTTP/1.1 200 OK
+Date: Thu, 09 Nov 2010 14:49:00 GMT
+Content-Length: 10
+
+contents2
+</data1>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+http
+</server>
+ <name>
+HTTP form posts with handle reset
+ </name>
+ <command>
+http://%HOSTIP:%HTTPPORT/2073 -F 'name=a;filename=a.pdf' --next http://%HOSTIP:%HTTPPORT/2073 -F 'name=b;filename=b.jpg'
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<strip>
+^(User-Agent:|Content-Type: multipart/form-data;|------------).*
+</strip>
+<protocol>
+POST /2073 HTTP/1.1
+Host: %HOSTIP:%HTTPPORT
+Accept: */*
+Content-Length: 189
+
+Content-Disposition: form-data; name="name"; filename="a.pdf"
+Content-Type: application/pdf
+
+a
+POST /2073 HTTP/1.1
+Host: %HOSTIP:%HTTPPORT
+Accept: */*
+Content-Length: 184
+
+Content-Disposition: form-data; name="name"; filename="b.jpg"
+Content-Type: image/jpeg
+
+b
+</protocol>
+</verify>
+</testcase>