Enable kernel module versioning

Closes #14 - Restore support for kernel module versioning in config (MODVERSIONS=y)
Update README for this in patches/kernel-4.19-cs
-- changes for mbuf, gpstime symbol versions --
Remove mbuf, gpstime symbols from src/drv/ExtraSymbols.symvers
Add caching of Module.symvers in /var/cache/(mbuf,gpstime) to src/drv/(mbuf,gpstime)/Makefile
Update RCG autoconf (rename configure.in to configure.ac, add bootstrap, remove configure)
Add test, path variables for Module.symvers files for mbuf,gpstime to configure.ac
Add mbuf,gpstime Module.symvers paths to src/epics/util/Makefile.in
Add KBUILD_EXTRA_SYMBOLS statements to front-end make file in src/epics/util/feCodeGen.pl
-- changes to front-end kernel module symbol versions --
Remove need_to_load_IOP_first symbol from src/drv/ExtraSymbols.symvers
Add caching of need_to_load_IOP_first symbol version, copy of cache for builds to src/epics/util/Makefile.in
Add KBUILD_EXTRA_SYMBOLS for this to src/epics/util/feCodeGen.pl