misc/cstring.h: simplify implementation of b_cstring_memchr() using the character iteration macro
misc/cstring.h: simplify implementation of b_cstring_memchr() using the character iteration macro

DavidCox1979/badvpn,DavidCox1979/badvpn,DavidCox1979/badvpn,DavidCox1979/badvpn,DavidCox1979/badvpn