Make color escape sequences conform to terminfo

powerline can combine multiple 256color rendition changes in the same
SGR sequence, e.g., `ESC [ 48;5;179; 38;5;100 m` to change both the
background and foreground color.

The xterm-256color terminfo entry requires (and mosh enforces) that
these be in separate escapes. This commit makes the powerline output
render in mosh.

Example: `echo -e "setab 179\nsetaf 100" | tput -S | od -c` generates
`ESC [ 48;5;179 m ESC [ 38;5;100 m`.

* https://github.com/keithw/mosh/issues/507
* https://github.com/keithw/mosh/issues/110#issuecomment-5053997