Don't use alias in travis.sh.

We don't want to clobber anyone's make alias when running travis.sh
directly.
Make `EventType` enum private to input.c

This enum doesn't need to be public since `event_poll` is only interested in
user input(but other events may be handled by libuv callbacks).
Re-integrate FEAT_FILTERPIPE code

This feature was accidentally removed when doing the initial import from vim. It
makes vim use pipes instead of temporary files for filtering buffers through
shell commands.

I found that this was missing when looking for references of
SHELL_READ/SHELL_WRITE outside mch_call_shell`.

When `mch_call_shell` is reimplemented on top of libuv process management
facilities, pipes will always be used for communication with child processes so
it makes sense to enable the feature permanently.
Rename mch_* functions to os_* in os module.
Address clint warnings and other style issues.

* Add const.
* Fix conditions (move && from end to start of line).
* Use int32_t instead of long.
* Use //-style comments.