watchcat: add support for TCP port checking

Maintainer: @mips171 (original Roger D <rogerdammit [at] gmail.com>)

Description:

This is an idea to incorporate in the watchcat package the support to check for open TCP ports. The reasons are:

• This package is very small (few text/script files), so it doesn't depends on third party large binaries/libraries.
• The tool has support to execute user scripts. Therefore it could be useful to execute some actions (restart a service for example) if some TCP port is not active (aka not opened).

Based on this I have the objective of use the netcat applet from the busybox tool (nc -z <addr>:<port>) for checking if a TCP port is open. To implement this we only need to add a new "ping" mode and add the port value, and add some code in the script.

My proposal for the configuration:

config watchcat
        option mode 'run_script'
        option script '/etc/watchcat.user.sh'
        option period '6h'
        option pinghosts '127.0.0.1'
        option pingperiod '30s'
        option pingsize 'tcpport'  # new value
        option pingport '80'       # new option

And then regarding the script code it will be necessary only to execute the command nc -z ${pinghosts}:${pingport} instead of the the ping if the "pingsize" option is "tcpport".

You agree with this idea?

Start by locating the watchcat package and its script code, then read the proposed configuration options and the existing ping behavior. Check how the BusyBox nc command is invoked and how ping failures trigger user scripts. Done means supporting the tcpport value and pingport option while preserving existing modes and checking the documented TCP endpoint.