Reformat the installation notes in Markdown. [skip ci]

(cherry picked from commit efaa145475)

INSTALL.md

@@ -1,49 +1,52 @@
+# tcpdump installation notes
 If you have not built libpcap, and your system does not have libpcap
 installed, install libpcap first.  Your system might provide a version
 of libpcap that can be installed; if so, to compile tcpdump you might
 need to install a "developer" version of libpcap as well as the
 "run-time" version.  You can also install tcpdump.org's version of
-libpcap; see the README.md file in this directory for the location.
+libpcap; see [this file](README.md) for the location.
 
 You will need a C99 compiler to build tcpdump.  The build system
 will abort if your compiler is not C99 compliant.  If this happens, use
 the generally available GNU C compiler (GCC) or Clang.
 
-After libpcap has been built (either install it with "make install" or
+After libpcap has been built (either install it with `make install` or
 make sure both the libpcap and tcpdump source trees are in the same
-directory), run ./configure (a shell script).  "configure" will
-determine your system attributes and generate an appropriate Makefile
-from Makefile.in.  Now build tcpdump by running "make".
+directory), run `./configure` (a shell script). `configure` will
+determine your system attributes and generate an appropriate `Makefile`
+from `Makefile.in`.  Now build tcpdump by running `make`.
 
-If everything builds ok, su and type "make install".  This will install
+If everything builds ok, `su` and type `make install`.  This will install
 tcpdump and the manual entry.  Any user will be able to use tcpdump to
 read saved captures.  Whether a user will be able to capture traffic
-depends on the OS and the configuration of the system; see the tcpdump
-man page for details.  DO NOT give untrusted users the ability to
+depends on the OS and the configuration of the system; see the
+[tcpdump man page](https://www.tcpdump.org/manpages/tcpdump.1.html)
+for details.  DO NOT give untrusted users the ability to
 capture traffic.  If a user can capture traffic, he or she could use
 utilities such as tcpdump to capture any traffic on your net, including
 passwords.
 
 Note that most systems ship tcpdump, but usually an older version.
 Building tcpdump from source as explained above will usually install the
-binary as "/usr/local/bin/tcpdump".  If your system has other tcpdump
+binary as `/usr/local/bin/tcpdump`.  If your system has other tcpdump
 binaries, you might need to deinstall these or to set the PATH environment
-variable if you need the "tcpdump" command to run the new binary
-("tcpdump --version" can be used to tell different versions apart).
+variable if you need the `tcpdump` command to run the new binary
+(`tcpdump --version` can be used to tell different versions apart).
 
 If your system is not one which we have tested tcpdump on, you may have
-to modify the configure script and Makefile.in. Please send us patches
+to modify the `configure` script and `Makefile.in`. Please
+[send us patches](https://www.tcpdump.org/index.html#patches)
 for any modifications you need to make.
 
-Please see "README.md" for notes about tested platforms.
+Please see [this file](README.md) for notes about tested platforms.
 
 
-FILES
------
+## Description of files
+```
 CHANGES		- description of differences between releases
 CONTRIBUTING	- guidelines for contributing
 CREDITS		- people that have helped tcpdump along
-INSTALL.txt	- this file
+INSTALL.md	- this file
 LICENSE		- the license under which tcpdump is distributed
 Makefile.in	- compilation rules (input to the configure script)
 README.md	- description of distribution
@@ -121,3 +124,4 @@ timeval-operations.h - timeval operations macros
 udp.h		- UDP definitions
 util-print.c	- utility routines for protocol printers
 win32		- headers and routines for building on Win32 systems
+```

Makefile.in

@@ -336,7 +336,7 @@ EXTRA_DIST = \
 	CONTRIBUTING \
 	CREDITS \
 	CMakeLists.txt \
-	INSTALL.txt \
+	INSTALL.md \
 	LICENSE \
 	Makefile.in \
 	Makefile-devel-adds \