Fix crash when system reports huge load averages

[zlandau]

The load average reporting functions in showsys.c use static buffer
sizes. When the load averages on a machine are very large, this causes
the writes to extend past the buffer. With this commit, if a number is
too large then we just show '>NNNNNN'. I'm not sure if this is the best
choice, so I'm open to other ideas.

This is what the output looks like when we exceed the maximums:

CPL | avg1 >999999 | avg5 >999999 | avg15 >99999 | csw 103117e3 | intr 88296e3 |
Note that this was triggered from a kernel that is reporting clearly
inaccurate numbers:

$ cat /proc/loadavg
1.25 2.40 368567045.47 1/589 53576

But regardless, crashing is no fun.

For future reference, I narrowed down the issue by building with
-fsanitize=address. For example:

  ==55396==ERROR: AddressSanitizer: global-buffer-overflow on address 0x55c527d6f14f at pc 0x7f4729942df9 bp 0x7ffe1fd30710 sp 0x7ffe1fd2fea0
  WRITE of size 10 at 0x55c527d6f14f thread T0
      # 0 0x7f4729942df8 in __interceptor_vsprintf /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:1627
      # 1 0x7f47299432cf in __interceptor_sprintf /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:1670
      # 2 0x55c527d1c167 in sysprt_CPLAVG15 (/home/kapheine/projects/atop/atop+0x74167)
      # 3 0x55c527d2087a in showsysline (/home/kapheine/projects/atop/atop+0x7887a)
      # 4 0x55c527d1471f in prisyst (/home/kapheine/projects/atop/atop+0x6c71f)
      # 5 0x55c527d090ff in generic_samp (/home/kapheine/projects/atop/atop+0x610ff)
      # 6 0x55c527ce17c9 in main (/home/kapheine/projects/atop/atop+0x397c9)
      # 7 0x7f472952a022 in __libc_start_main (/usr/lib/libc.so.6+0x27022)
      # 8 0x55c527ce266d in _start (/home/kapheine/projects/atop/atop+0x3a66d)

  0x55c527d6f14f is located 49 bytes to the left of global variable 'buf' defined in 'showsys.c:981:54' (0x55c527d6f180) of size 15
  0x55c527d6f14f is located 0 bytes to the right of global variable 'buf' defined in 'showsys.c:1000:54' (0x55c527d6f140) of size 15

[Atoptool]

Thanks!