CLI help updates

[dra27]

Best checked commit-by-commit:

• Use Cmdliner.Manpage.s_ constants instead of hard-coded section names (small tidy-up, although possibly one "bug" found)
• Aliases put the --help and --version options in OPTIONS instead of COMMON OPTIONS
• opam lock --help was missing all the common information (ENVIRONMENT, BUGS, etc.)

There are two other things to consider/fix:

• We call the section ENVIRONMENT VARIABLES (OpamArg.help_sections) but the standard name is ENVIRONMENT (Cmdliner.Manpage.s_environment) - any reason not to change this?
• opam lock has a section What is changed in the lock file? - this should be in capitals and re-worded. Perhaps LOCK PROCEDURE, perhaps?

[dra27]

1e5c401 FWIW should go to 2.0 as well.

[rjbou]

On the environment variable, no reason to not switch to Cmdliner.Manpage.s_environment.
For opam lock, it is not really the procedure, but what contains the lock file regarding the regular opam file, maybe something like LOCK FILE CONTENT?

[dra27]

How about LOCK FILE UPDATES? "CONTENT" to me implied that it completely writes the file, but I (think?!) the point is that it makes changes to it, right?

[rjbou]

ok with that, or a more specific LOCK FILE UPDATED FIELDS, to not let think that it is updates on the lock file process, but the fields that are updated from the opam file.

[AltGr]

We call the section ENVIRONMENT VARIABLES (OpamArg.help_sections) but the standard name is ENVIRONMENT (Cmdliner.Manpage.s_environment) - any reason not to change this?

Only reason I could see is the completion scripts that parse the manpage output; but they don't use this section so it should be fine.