various bugs

[grinapo]

I am not sure there's a point reporting bugs, since this project seems to be dormant at best. I just summarize.

• debug masks sometimes get lost, for example grok_config.c never logs for me, which is bad for filename errors (or, actually, config errors)
• plenty of segfaults due to non-checked return values from extrenal libs, mainly libpcre
  ** for eample grok_match_walk_next accepts position 1852178432 happily for a 80 characters long string - and also crashes happily immediately after.
• libpcre seems to limit regexp length, none of my (test) grok files was below the limit. :-)
• non-follow file read doesn't work; I haven't checked how it was designed, but definitely not as expected.
• Makefile clean does not clean libgrok* and discogrok binaries

[grinapo]

Added PR #37 for the bounds check bugging me.