If attempting to turn memory-mapped access on fails for any reason other

If attempting to turn memory-mapped access on fails for any reason other
than "the kernel doesn't support memory-mapped access to PF_PACKET
sockets", treat that as an error.  If it fails for that reason, don't
leave gunk behind in the pcap_t's error buffer.

Clean up the error messages a bit (the result of strerror() suffices; we
don't need the numeric value of errno, nor do we need the file
descriptor number of the socket on which we're working).