parser using yacc(1). This is a major change but is clearer and simpler
and allows some edge cases to be made more consistent, as well as
tidying up how aliases are handled. It will also allow some further
improvements later.

Entirely the same parser is now used for parsing the configuration file
and for string commands. This means that constructs previously only
available in .tmux.conf, such as %if, can now be used in string commands
(for example, those given to if-shell - not commands invoked from the
shell, they are still parsed by the shell itself).

The only syntax change I am aware of is that #{} outside quotes or a
comment is now considered a format and not a comment, so #{ is now a
syntax error (notably, if it is at the start of a line).

This also adds two new sections to the man page documenting the syntax
and outlining how parsing and command execution works.

Thanks to everyone who sent me test configs (they still all parse
without errors - but this doesn't mean they still work as intended!).

Thanks to Avi Halachmi for testing and man page improvements, also to
jmc@ for reviewing the man page changes.

multiple commands to be easily bound to one hook. set-hook and
show-hooks remain but they are now variants of set-option and
show-options. show-options now has a -H flag to show hooks (by default
they are not shown).

Hunt in GitHub issue 1443.

cmd_string_parse. Reported by Jimi Damon in GitHub issue 975.

contains items of the form "alias=command". This is consulted when an
unknown command is parsed.

just return NULL.