[BASH] Add function add_MANPATH_if_exists.

Signed-off-by: Wei-Chung Wen <morisato3711@gmail.com>
Merge branch 'master' into MacBookAir

Signed-off-by: Wei-Chung Wen <morisato3711@gmail.com>
[BASH] Set PATH and MANPATH for MacPorts.

Signed-off-by: Wei-Chung Wen <morisato3711@gmail.com>