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
Prior to this fix, a function_block stored a process_t, which was only used
when printing backtraces. Switch this to an array of arguments, and make
various other cleanups around null terminated argument arrays.
This broke fishtape, which did
somestuff | fish -c "source"
Because `source` didn't have a redirection, it refused to read from
stdin.
So, to keep the common issue of `source (command that does not print)`
from seeminly stopping fish, we instead actually check if stdin is a terminal.
For things like
source $undefined
or
source (nooutput)
it was quite annoying that it read from tty.
Instead we now require a "-" as the filename to read from the tty.
This does not apply to reading from stdin if it's redirected, so
something | source
still works.
Fixes#2633.
