xfer: add new config option file.xfer.auto_crc32

If a filename has a CRC32 tag in it, the file is checksumed while
being downloaded. The result of the checksum match is displayed in the
xfer buffer.
xfer: Specify what checksum matched or mismatched
xfer: Refine crc32 regex
xfer: NULLify xfer_crc32_preg on init failure
xfer: further refine crc32 regex
core: Specify that the conversion is to Kibi/Mibi/Gibi bytes
core: Add timeout to network_connect
xfer: Make socket nonblocking before connect
xfer: Provide uniform infolists for signal callbacks and infolist hook
core: Bump connect timeout to 14 seconds