src/collectd.c: Set the LC_NUMERIC locale to "C".

When using ASCII protocols, such as the Graphite protocol, doubles are
converted to strings using sprintf(3) and parsed with strtod(3). If the
effective locale uses something different from a dot (".") as the
decimal separator (e.g. Portuguese, German), this breaks.

This patch sets the LC_NUMERIC locale to "C" on start-up, so that
strings are produced and parsed in a well-defined manner.

Fixes: #512
Move collectd daemon files to the src/daemon/ directory.
common.[ch]: Move escape_string() to here.
Move utils_parse_option.[ch] out of the daemon/ directory.
COPYING: Add the MIT License and add a note about plugins being licensed individually.