Remove all generated object files on "make clean"
Separate overridable part of CFLAGS

An external build system might want to select for example a different
optimization level, but overriding CFLAGS would also drop "-std=c99"
and that breaks compilation. Fixed by using two separate Make variables
for overridable and non-overridable compiler flags.
Use strncpy instead of strlcpy to copy splashkill process name

In this particular use, the fact that strncpy doesn't guarantee
a terminating '\0' when the string only just fits is actually an
advantage, since the original string already has a '\0' that is
not included in the passed size.

Also, strlcpy is not available in glibc. It is available in uClibc
and musl though.
Replaced _BSD_SOURCE with _DEFAULT_SOURCE

Recent glibc versions warn about _BSD_SOURCE, while uClibc and musl
support either feature test macro.
Add license information

I checked with all other authors (Ignacio and Paul) and they agreed to
use the 2-clause BSD license.
Bump version to 2.0.1