mkindex.pl improvements

[sluicing]

Improve index generation

• Update Makefile to regenerate 'uni.md' if the 'bookindex.md' file has changed. ( If there's a reason the Makefile is this way, let me know and I can undo this)

• Improve 'mkindex.pl' to tTrack whether a code block with ``` has been seen, to avoid treating '# foo' comments within code blocks as sections to link to in indexes. This fixes such bad index links as
  "clone: Install GCC for x86_64,"

• Improve 'mkindex.pl' to strip more characters that aren't legal in CSS Ids: those IDs should begin with a letter or an underscore, and should contain only letters, numbers, underscore or dash after the first char. This fixes a few IDs that contains '?', '', '!'

• Improve 'mkindex.pl' support for header sections with explicit section IDs:

  • If a header section includes an explicit section ID, link to that instead of generating one from the header text
  • Include an explicit section id, '{#everything-curl-index}', at the top of the bookindex.md file

• Update 'bookindex.md' with regenerated index from above improvements to 'mkindex.pl'

Notes:

• See https://stackoverflow.com/questions/448981/which-characters-are-valid-in-css-class-names-selectors for discussion of legal CSS class names
• The support for explicit section IDs was motivated somewhat by work to generate working in-doc links for the PDF; I think including the occasional explicit section ID might improve / work around errors in the HTML version of the index too, but I haven't investigated in any detail.

[bagder]

Can you please rebase and force-push this? I'm afraid it has a merge conflict now.

[sluicing]

Can you please rebase and force-push this? I'm afraid it has a merge conflict now.

Fixed; let me know if you'd like other small tweaks to the PR

[bagder]

Thanks!