Fix uptime matching pcre_compile call and reset errno.

pcre_compile requires errptr to be present, and we should not tie the
match to the beginning of the line.

Additionally, reset errno before we try to call any uptime functions,
since otherwise we may get an error message even when there is no
error.
Merge pull request #814 from kacfengine/uptime_pcre_fix

Fix uptime matching pcre_compile call and reset errno.