output: add mustach templating support

Based on José Bollo's mustache C implementation.  This adaptation uses
project specific data structures to avoid overhead with json parsing.

Reference: https://gitlab.com/jobol/mustach.git
Commit: d84608a69033d38c81b8fcff0cb272e225dd5428
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

man/dhcpd-pools.1.in

@@ -8,6 +8,7 @@ dhcpd-pools \- ISC dhcpd pools usage analysis
 .OP \-\-sort nimcptTe
 .OP \-\-reverse
 .OP \-\-format tHcxXjJ
+.OP \-\-mustach template
 .OP \-\-output file
 .OP \-\-limit nr
 .OP \-\-color when
@@ -103,6 +104,14 @@ to include ethernet address.
 The default format is
 .IR @OUTPUT_FORMAT@ .
 .TP
+\fB\-\-mustach\fR=\fITEMPLATE\fR
+Output using mustach
+.I template
+file.  Mustache tags in the template are same as json output without IP
+address information.  When the native output formats controlled with
+.B \-\-format
+option do not provide what you need you should use mustach instead.
+.TP
 \fB\-o\fR, \fB\-\-output\fR=\fIFILE\fR
 .I File
 where output is written.  Default is stdout.
@@ -280,8 +289,11 @@ Original design by Sami Kerola.
 XML support by Dominic Germain, Sogetel inc.
 .br
 IPv6 support by Cheer Xiao.
-.PP
-The software has FreeBSD License.
+.br
+Mustache templating support by José Bollo.
+.SH LICENSE
+The dhcpd-pools uses FreeBSD License, the mustache uses Apache License, and
+the gnulib modules are Mostly, but not entirely, GPL.
 .SH "REPORTING BUGS"
 Report bugs to
 .MT @PACKAGE_BUGREPORT@
@@ -294,4 +306,6 @@ Home page
 .SH "SEE ALSO"
 .BR dhcpd.leases (5),
 .BR dhcpd.conf (5),
-.BR chmod (1)
+.BR chmod (1),
+.UR https://mustache.github.io/
+.UE