use os_bsd and OS_BSD and add some OS_FREEBSD conditions

this change is the first part of the FreeBSD support

BUG=
TEST=


Review URL: http://codereview.chromium.org/8773051

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@112936 0039d316-1c4b-4281-b951-d872f2087c98

• base/base.gypi[diff]
• base/debug/debugger_posix.cc[diff]
• base/process_util.h[diff]
• base/process_util_freebsd.cc[Added - diff]
• base/process_util_posix.cc[diff]
• build/common.gypi[diff]