Be more library friendly, by not aborting in library errors.
spice_common now includes a proper log handler that will abort by
default when reaching a warning.
SPICE_ABORT_LEVEL can be changed to modify run-time abort level.
SPICE_DEBUG_LEVEL can be changed to be more verbose. By default, only
log level more importants than WARNING.
Only memory-related functions are allowed to abort(), since they are
not recoverable errors in the library.
When using config.h, it must be the very first include in all source
files since it contains #define that may change the compilation process
(eg libc structure layout changes when it's used to enable large file
support on 32 bit x86 archs). This commit adds it at the beginning
of all .c and .cpp files
