This commit uses options and flags that will work on both the BSD and
the `man-db` implementations, however those changes remain unportable as
the POSIX standard only defines a single `-k` flag for the utility,
which we don't need.
The call to the `col` utility has also been replaced by a call to `sed`,
as the former is only shipped on systems that have the `nroff` formatter
installed.
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`)
