MFC: r287013 (partial), r294616, r314097 (partial)

• Allow overriding the FDT slicer with a custom slicer.
• Teach the flashmap code about SPI flash.
• Allow different slicers for different flash types to be registered with geom_flashmap(4) and teach it about MMC for slicing enhanced user data area partitions. The FDT slicer still is the default for CFI, NAND and SPI flash on FDT-enabled platforms.
• In addition to a device_t, also pass the name of the GEOM provider in question to the slicers as a single device may provide more than one provider.
• Build a geom_flashmap.ko.
• Use MODULE_VERSION() so other modules can depend on geom_flashmap(4).
• Remove redundant/superfluous GEOM routines that either do nothing or provide/just call default GEOM (slice) functionality.
• Trim/adjust includes