OVS_COLORS environment variable is parsed to extract user-defined
preferences regarding colors (this is used to set up a color theme, not
to replace the `--color` option for activating color output).

The string should be of a format similar to LS_COLORS or GREP_COLORS,
with available colors being as follows:

* ac: action field
* dr: drop keyword
* le: learn keyword
* pm: parameters receiving attributes
* pr: keyword having parenthesis
* sp: some special keywords
* vl: lone values with no parameter name

For color whose idendifier does not appear in the string, the default
hardcoded value is used instead.

As an example, setting OVS_COLORS to the following string is equivalent
to using the default values:

    OVS_COLORS="ac:01;31:dr=34:le=31:pm=36:pr=35:sp=33:vl=32"

Signed-off-by: Quentin Monnet <quentin.monnet@6wind.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>