Add support for extra sendmail arguments (#2731)

* Add support for extra sendmail arguments

* Sendmail args to exec.command should be a list

* Add go-shellquote package

* Use go-shellquote lib for parsing Sendmail args

* Only parse if sendmail is configured

vendor/github.com/kballard/go-shellquote/quote.go

@@ -0,0 +1,102 @@
+package shellquote
+
+import (
+	"bytes"
+	"strings"
+	"unicode/utf8"
+)
+
+// Join quotes each argument and joins them with a space.
+// If passed to /bin/sh, the resulting string will be split back into the
+// original arguments.
+func Join(args ...string) string {
+	var buf bytes.Buffer
+	for i, arg := range args {
+		if i != 0 {
+			buf.WriteByte(' ')
+		}
+		quote(arg, &buf)
+	}
+	return buf.String()
+}
+
+const (
+	specialChars      = "\\'\"`${[|&;<>()*?!"
+	extraSpecialChars = " \t\n"
+	prefixChars       = "~"
+)
+
+func quote(word string, buf *bytes.Buffer) {
+	// We want to try to produce a "nice" output. As such, we will
+	// backslash-escape most characters, but if we encounter a space, or if we
+	// encounter an extra-special char (which doesn't work with
+	// backslash-escaping) we switch over to quoting the whole word. We do this
+	// with a space because it's typically easier for people to read multi-word
+	// arguments when quoted with a space rather than with ugly backslashes
+	// everywhere.
+	origLen := buf.Len()
+
+	if len(word) == 0 {
+		// oops, no content
+		buf.WriteString("''")
+		return
+	}
+
+	cur, prev := word, word
+	atStart := true
+	for len(cur) > 0 {
+		c, l := utf8.DecodeRuneInString(cur)
+		cur = cur[l:]
+		if strings.ContainsRune(specialChars, c) || (atStart && strings.ContainsRune(prefixChars, c)) {
+			// copy the non-special chars up to this point
+			if len(cur) < len(prev) {
+				buf.WriteString(prev[0 : len(prev)-len(cur)-l])
+			}
+			buf.WriteByte('\\')
+			buf.WriteRune(c)
+			prev = cur
+		} else if strings.ContainsRune(extraSpecialChars, c) {
+			// start over in quote mode
+			buf.Truncate(origLen)
+			goto quote
+		}
+		atStart = false
+	}
+	if len(prev) > 0 {
+		buf.WriteString(prev)
+	}
+	return
+
+quote:
+	// quote mode
+	// Use single-quotes, but if we find a single-quote in the word, we need
+	// to terminate the string, emit an escaped quote, and start the string up
+	// again
+	inQuote := false
+	for len(word) > 0 {
+		i := strings.IndexRune(word, '\'')
+		if i == -1 {
+			break
+		}
+		if i > 0 {
+			if !inQuote {
+				buf.WriteByte('\'')
+				inQuote = true
+			}
+			buf.WriteString(word[0:i])
+		}
+		word = word[i+1:]
+		if inQuote {
+			buf.WriteByte('\'')
+			inQuote = false
+		}
+		buf.WriteString("\\'")
+	}
+	if len(word) > 0 {
+		if !inQuote {
+			buf.WriteByte('\'')
+		}
+		buf.WriteString(word)
+		buf.WriteByte('\'')
+	}
+}