lib: move sort and search from strfuncs.h to sort.h

lib: extract sort-helpers into separate sort.h file The macro definition was nothing to do with strings, and we can put trivial common-type comparators here. They didn't need to be static inline, as they can never be inlined anyway, being only used via function pointers, but that preserves the closest equivalent to the current code. Signed-off-by: Phil Carmody <phil@dovecot.fi>