curl: two new command line options for etags

--etag-compare and --etag-save

Suggested-by: Paul Hoffman
Fixes #4277
Closes #4543

docs/cmdline-opts/Makefile.inc

@@ -38,6 +38,8 @@ DPAGES =					\
   dump-header.d					\
   egd-file.d					\
   engine.d					\
+  etag-save.d                   \
+  etag-compare.d                \
   expect100-timeout.d				\
   fail-early.d					\
   fail.d					\

docs/cmdline-opts/etag-compare.d

@@ -0,0 +1,17 @@
+Long: etag-compare
+Arg: <file>
+Help: Pass an ETag from a file as a custom header
+Protocols: HTTP
+---
+This option makes a conditional HTTP request for the specific
+ETag read from the given file by sending a custom If-None-Match
+header using the extracted ETag.
+
+For correct results, make sure that specified file contains only a single
+line with a desired ETag. An empty file is parsed as an empty ETag.
+
+Use the option --etag-save to first save the ETag from a response, and
+then use this option to compare using the saved ETag in a subsequent request.
+
+\fCOMPARISON\fP: There are 2 types of comparison or ETags, Weak and Strong.
+This option expects, and uses a strong comparison.

docs/cmdline-opts/etag-save.d

@@ -0,0 +1,15 @@
+Long: etag-save
+Arg: <file>
+Help: Parse ETag from a request and save it to a file
+Protocols: HTTP
+---
+This option saves an HTTP ETag to the specified file. Etag is
+usually part of headers returned by a request. When server sends an
+ETag, it must be enveloped by a double quote. This option extracts the
+ETag without the double quotes and saves it into the <file>.
+
+A server can send a week ETag which is prefixed by "W/". This identifier
+is not considered, and only relevant ETag between quotation marks is parsed.
+
+It an ETag wasn't send by the server or it cannot be parsed, and empty
+file is created.