update pre/post script to only kill process named watchman, not .*watchman.*

Summary:
pkill -9 watchman would kill any processes that has watchman in it named.
when using alien to convert the rpm created by `arc package`, scripts called
fb-watchman.prerm and fb-watchman.postinst are ran and would kill themselves
preventing the install to succeed

Test Plan:
$ pgrep watchman
22989
┌─[09:55:07]──[chantra]──[dev421]:~/fbcode $
└──(master) >>
$ pkill -9 -x watchman || /bin/true
┌─[09:55:10]──[chantra]──[dev421]:~/fbcode $
└──(master) >>
$ pgrep watchman
┌─[09:55:12]──[chantra]──[dev421]:~/fbcode $
└──[Exit 1] (master) >>
$ pgrep watchman
28386
...
...
$ pgrep watchman
28386
┌─[09:59:13]──[chantra]──[dev421]:~/fbcode $
└──(master) >>
$ pkill -x watchman || /bin/true
┌─[09:59:19]──[chantra]──[dev421]:~/fbcode $
└──(master) >>
$ pgrep watchman
┌─[09:59:21]──[chantra]──[dev421]:~/fbcode $
└──[Exit 1] (master) >>

Reviewers: wez

Reviewed By: wez

CC: royw, sid0

Differential Revision: https://phabricator.fb.com/D1254095

Task ID: 4049275