Every builtin or function shipped with fish supports flag -h or --help to
print a slightly condensed version of its manpage.
Some of those help messages are longer than a typical screen;
this commit pipes the help to a pager to make it easier to read.
As in other places in fish we assume that either $PAGER or "less" is a
valid pager and use that.
In three places (error messages for bg, break and continue) the help is
printed to stderr instead of stdout. To make sure the error message is
visible in the pager, we pass it to builtin_print_help, every call of which
needs to be updated.
Fixes#6227
This makes the following changes:
1. Events in background threads are executed in those threads, instead of
being silently dropped
2. Blocked events are now per-parser instead of global
3. Events are posted in builtin_set instead of within the environment stack
The last one means that we no longer support event handlers for implicit
sets like (example) argv. Instead only the `set` builtin (and also `cd`)
post variable-change events.
Events from universal variable changes are still not fully rationalized.
