make prettyping work with any posix-compatible shell

[xwang1498]

Because most of the heavy lifting is done in awk, we actually don't need any bash-specific features to run prettyping. Making prettyping work on any posix-compatible shell will extend this great program's reach even further!

This PR builds on the work done in PR #44 , and therefore supersedes and closes #44 and closes #40.

Tested on various shells, including:

• bash 3.2
• bash 5.3
• dash 0.5.12
• ksh 93u+ 2021-08-01
• ksh 93u+m 1.0.10
• Busybox ash v1.36.1
• Almquist shell (FreeBSD default)

Tested on various operating systems, including:

• macOS 13
• OpenBSD 7.6
• FreeBSD 13.5
• Ubuntu 22.04
• OpenWrt (still requires changes in PR Improve support for embedded systems like OpenWrt #42)
• Cygwin

Implementation notes, which may be interesting while reviewing this PR:

• Posix sh doesn't support arrays, but we can imitate them by working on the psuedo-array $@.
• When parsing arguments for ping, we don't need to track them in a separate array, but can just append the non-processed ones to the end of $@. This way, they'll circle around to the front. For simplicity, parse them globally rather than inside of parse_arguments function (which would have its own $@).
• Notes from PR better handling of SIGINT / Ctrl-C #44 apply re. signal handling.
• Some shells (dash) interrupt read with the SIGWINCH, others do not. So we need to restart read if it was interrupted by SIGWINCH, but not if the input is finished. I added a SIGWINCH test case to mockping.sh.