Fix support for backslash-newline in double quotes in unescape functions. Makes tests pass
again.
more detailed debugging when terminal type cannot be found

Ref: #1060
fall back to ANSI terminal type if setupterm fails

Closes: https://github.com/fish-shell/fish-shell/issues/1060

'ansi' should always be present (tested on Solaris, Linux, FreeBSD,
Darwin).

Also overrides TERM so that other programs behave consistently e.g.
fish_pager.

The error message makes no specific mention of terminfo or termcap as
these vary across operating systems.

(r+ @ridiculousfish with thanks)
Fix for annoying error message when converting out of range escape
https://github.com/fish-shell/fish-shell/issues/1107
Add missing const cast for setup_term
https://github.com/fish-shell/fish-shell/issues/1142
Merge remote-tracking branch 'upstream/master'