Separate flat, protected, and kernel mode library selected into separate files.  The conditional logic as exceed my able to comprehend and maintain the selections.

Various changes/fixes to get configs/stm32f4discovery/kostest working after the big configuration renaming (and after a long period of bit rot)

Fix hard coded values in dispatch_sysall inline assembly. Back out/corect part of last change; that was going the wrong direction.

Fix ARM7/9 and Cortex-A SYSCALLs:  For threads in SVC mode, the SVC instructions clobbers R14.  This must be taken account in the inline assembly

Rename  arch/arm/src/armv7-a/syscall.h to svcall.h to work around some include path name collisions; fix some compilation errors in SYSCALL logic when debug is enabled

Trick the top-level makefiles into building the SYSCALL proxy library but then not actually linking with it

include/sys/syscall.h: System definitions should depend on CONFIG_LIB_SYSCALL not CONFIG_NUTTX_KERNEL

nuttx/sched:  Remove explicit references to errno.  That is a problem from within the kernel for certain configurations

nuttx/sched:  Remove explicit references to errno.  That is a problem from within the kernel for certain configurations