misc: Replace many (but not all) instances of 'cgminer' with 'sgminer'.

Just ran a `sed -i 's/cgminer/sgminer/'` on several files.
build: fix warning + other small fixes.
doc: reference veox/sgminer in ChangeLog.
core: fix build error due to CGMINER_PREFIX reference.
misc: Replace all remaining instances of 'cgminer' with 'sgminer'.

sed again.