`tmux` will start new processes (e.g. when creating panes or windows)
with the same environment it was started with, which means that if the
$TMPDIR variable was overriden for the kakoune server from within
`tmux`, newly created panes/windows won't have access to the server
socket to sustain a session.
This commit fixes the issue by always exporting the $TMPDIR variable
from the parent `tmux` environment to the new processes.
Fixes#1319
Level out the builtin commands loaded at startup in terms of format and
expressiveness. The following convention was followed:
* commands that take more than one argument have to be described along
with their parameters prior to the actual documentation, otherwise the
docstring consists in a capitalized sentence
e.g. `command <arg1>: do something`
* optional arguments are enclosed in square brackets, to comply with the
format used for hardcoded commands
e.g. `cd [<directory>]`
* describe the effects of the command in the documentation string and
omit implementation details unless they are relevant. Usually command
names include the name of the tool they use, so they don't need to be
redundantly mentioned
e.g. `tmux-new-pane <arguments>: open a new pane`
* document the format the parameters to the commands, or list them if
they are to be chosen among a list of static values (c.f. `spell.kak`)
