MFC 287297,296236: Cleanups to cxgbetool.

MFC 287297,296236: Cleanups to cxgbetool.

287297:
- Replace N(a)/N(i)/N(T)/LEN(a)/ARRAY_SIZE(a) with nitems()
- Add missing <err.h> for err() and <sys/sysctl.h> for sysctlbyname()
- NULL -> 0 for 5th parameter of sysctlbyname()

Note, the original commit touched several files under tools/tools, but
this commit only includes changes to cxgbetool.

296236:
Fix some whitespace nits in cxgbetool.c.  No functional change.

Sponsored by:	Chelsio Communications

git-svn-id: svn://svn.freebsd.org/base/stable/10@308303 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f