first pass on automatically recording server-side demos.

setting sv_autoDemo 1 will enable.  you must explicitly set sv_autoDemoBots
1 to save bot demos automatically.  auto-demoing will interfere with manual
demoing, so don't mix the two.

the demos are grouped into a single folder per map + time map started.
optionally, set sv_autoDemoMaxMaps to automatically delete demos from more than N maps ago.
some svdemo cleanups.  fix bug where we wrote the wrong initial sequence
number causing start of demos to be wonky.
make sure we don't delta against a frame not written to the demo
bump up size of demoname to avoid truncation
use the same time format as CL_DemoFilename for default and autorecord demos
move the recursive rmdir to files.cpp, separate logic from FS_HomeRemove
clean up imports so only files.cpp needs the extra headers