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