This patch addes a "LICENSE" file, explaining our copyright status and
3-clause BSD license.

It also includes in documentation/LICENSE-* the licenses of the four projects
we copied code from - FreeBSD, Prex, Open Solaris and Musl. Our top-level
LICENSE file refers to these separate LICENSE files.

allows commands to specify a delimiter between tab completion
suggestions, the default is '\t' in case this function is not present
in the command interface.

the tab_pretty(arg) function is called just before printing an
autocompleted suggestion, it may be used by commands to prettify their
autocompletion output, will shortly be used by the cd command