Move config.h to include/nfsc and make use of it in nfsc/libnfs.h

<sys/time.h> was previously only included if ANDROID or AROS was defined which
seems a bit arbitrary as it's also required on other platforms (e.g. plain old
GNU/Linux).

Signed-off-by: Arne Redlich <arne.redlich@googlemail.com>
nfs_lseek{,_async}: allow negative offsets and guard against file positions < 0

Signed-off-by: Arne Redlich <arne.redlich@googlemail.com>