main: interpret only a double slash (//) as quoting, not a single one

Otherwise the first line of a multiline /*...*/-comment would be
seen as quoted and thus *not* as the first line of a paragraph.

In the code, use "/{2}" to prevent the remainder of the line
getting colored as a comment.

doc/nano.1

@@ -125,7 +125,7 @@ and place it at that position again upon reopening such a file.
 .BR "\-Q ""\fIregex\fB""" ", " "\-\-quotestr=""" \fIregex """
 Set the regular expression for matching the quoting part of a line.
 This is used when justifying.
-The default value is \fB"^([\ \\t]*[#/:>|}])+"\fP.
+The default value is \fB"^([\ \\t]*([#:>|}]|//))+"\fP.
 Note that \fB\\t\fP stands for an actual Tab.
 .TP
 .BR \-R ", " \-\-restricted

doc/nano.texi

@@ -221,7 +221,7 @@ and place it at that position again upon reopening such a file.
 @item -Q "@var{regex}"
 @itemx --quotestr="@var{regex}"
 Set the regular expression for matching the quoting part of a line, used
-when justifying.  The default value is @t{"^([@w{ }\t]*[#/:>|@}])+"}.
+when justifying.  The default value is @t{"^([@w{ }\t]*([#:>|@}]|//))+"}.
 Note that @code{\t} stands for a literal Tab character.
 
 @item -R
@@ -851,7 +851,7 @@ Obsolete option.  Recognized but ignored.
 @item set quotestr "@var{regex}"
 The email-quote string, used to justify email-quoted paragraphs.  This
 is an extended regular expression.  The default value is
-@t{"^([@w{ }\t]*[#/:>|@}])+"}.
+@t{"^([@w{ }\t]*([#:>|@}]|//))+"}.
 Note that @code{\t} stands for a literal Tab character.
 
 @item set rebinddelete

doc/nanorc.5

@@ -206,7 +206,7 @@ Obsolete option.  Recognized but ignored.
 .B set quotestr "\fIregex\fP"
 The email-quote string, used to justify email-quoted paragraphs.  This
 is an extended regular expression.  The default value is
-"\fB^([\ \\t]*[#/:>|}])+\fP".  Note that \fB\\t\fR stands for an actual
+"\fB^([\ \\t]*([#:>|}]|//))+\fP".  Note that \fB\\t\fR stands for an actual
 Tab character.
 .TP
 .B set rebinddelete

doc/sample.nanorc.in

@@ -128,7 +128,7 @@
 
 ## The email-quote string, used to justify email-quoted paragraphs.
 ## This is an extended regular expression.  The default is:
-# set quotestr "^([ 	]*[#/:>|}])+"
+# set quotestr "^([ 	]*([#:>|}]|//))+"
 
 ## Fix Backspace/Delete confusion problem.
 # set rebinddelete

src/nano.c

@@ -2470,7 +2470,7 @@ int main(int argc, char **argv)
 	if (brackets == NULL)
 		brackets = mallocstrcpy(NULL, "\"')>]}");
 	if (quotestr == NULL)
-		quotestr = mallocstrcpy(NULL, "^([ \t]*[#/:>|}])+");
+		quotestr = mallocstrcpy(NULL, "^([ \t]*([#:>|}]|/{2}))+");
 
 	/* Compile the quoting regex, and free it when it's good; otherwise,
 	 * retrieve and store the error message, to be shown when justifying. */