Make local BIT_STRING_BITNAME variables const

Make local BIT_STRING_BITNAME variables const

There's no reason for them not to be const. This is a piece of a larger
diff that I carry in several of my trees to move more things to rodata
or relro. The full diff requires a change to a public header and it's
very annoying to have to 'make includes' and recompile the entire lib
all the time when hopping from tree to tree.