Import libedit 2019-09-10

Compared to current version in base:

• great improvements on the Unicode support
• full support for filename completion including quoting which means we do not need anymore our custom addition)
• Improved readline compatiblity

Upgrading libedit has been a pain in the past, because somehow we never
managed to properly cleanup the tree in lib/libedit and each merge has always
been very painful. After years of fighting give up and refresh a merge from
scrarch properly in contrib.

Note that the switch to this version will be done in another commit.