server: terminal width should fallback to 80x25. (#1895)

Some environments might disable access to `/dev/tty`, fall
back to '80' in such scenarios.

Move to 'cheggaaa/pb' package for better cross platform
support on fetching terminal width.

Fixes #1891

vendor/github.com/cheggaaa/pb/termios_bsd.go

@@ -0,0 +1,9 @@
+// +build darwin freebsd netbsd openbsd dragonfly
+// +build !appengine
+
+package pb
+
+import "syscall"
+
+const ioctlReadTermios = syscall.TIOCGETA
+const ioctlWriteTermios = syscall.TIOCSETA