Improve index generation:

- Update Makefile to regenerate 'uni.md' if the 'bookindex.md' file has changed
- Improve 'mkindex.pl' to:
   - Add an explicit section id, '{#everything-curl-index}', to the top of the bookindex.md file
   - Track 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](get/win-msys2.md#install-gcc-for-x86_64),"
   - 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.  (See https://stackoverflow.com/questions/448981/which-characters-are-valid-in-css-class-names-selectors )  This fixes a few IDs that contains '?', '\', '!'
   - Sorting seems to have been corrected for items that reference "All options", but I didn't dig into how/why too much since the result is an improvement.
- Update 'bookindex.md' with regenerated index from above improvements to 'mkindex.pl'

Makefile

@@ -17,7 +17,7 @@ wordcheck:
 fixup:
 	for i in $(MDS); do ./lang.sh $$i; done
 
-uni.md: uni.pl $(MDS) SUMMARY.md
+uni.md: uni.pl $(MDS) SUMMARY.md $(OUT)
 	./uni.pl SUMMARY.md >$@
 
 everything-curl.pdf:	uni.md