Let inset take -C instead of -M

[PaulWessel]

Description of proposed changes

As discussed in #6288, I unfortunately selected -M as the option for adding interior margins for a map inset, even though -C as used in subplot is the most comparable situation. This PR rectifies this by switching inset to use -C but of course retaining backwards compatibility with any -M use. Some related points:

1. I ran all tests as they were first to ensure the -M options were correctly processed
2. I then switched the 4 scripts that used -M to -C and ran the tests again
3. Since the new -C has to also handle the syntax of -M it can now handle the old -C syntax plus -Cxmargin/ymargin and -Cwmargin/emargin/smargin/nmargin which is even more flexible than either previous option.
4. It also turned out that inset had been left out of the synopsis reformatting (to use terminal width) so I fixed that in the process.

All tests still pass as before, the documentation has been updated. Once this PR is approved and merged I will enhance the -C option in subplot to be equally flexible.

[seisman]

Looks good to me.