allow write callbacks to indicate OOM to libcurl

Allow (*curl_write_callback) write callbacks to return
CURL_WRITEFUNC_OUT_OF_MEMORY to properly indicate libcurl of OOM conditions
inside the callback itself.

docs/libcurl/curl_easy_setopt.3

@@ -160,6 +160,11 @@ From 7.18.0, the function can return CURL_WRITEFUNC_PAUSE which then will
 cause writing to this connection to become paused. See
 \fIcurl_easy_pause(3)\fP for further details.
 
+From 7.22.1, the function can return CURL_WRITEFUNC_OUT_OF_MEMORY to indicate
+libcurl that an attempt to dynamically allocate memory from within the write
+callback itself has failed. This will abort the transfer and make libcurl
+return CURLE_OUT_OF_MEMORY.
+
 This function may be called with zero bytes data if the transferred file is
 empty.
 

docs/libcurl/symbols-in-versions

@@ -685,4 +685,5 @@ CURL_VERSION_SPNEGO             7.10.8
 CURL_VERSION_SSL                7.10
 CURL_VERSION_SSPI               7.13.2
 CURL_VERSION_TLSAUTH_SRP        7.21.4
+CURL_WRITEFUNC_OUT_OF_MEMORY    7.22.1
 CURL_WRITEFUNC_PAUSE            7.18.0