strchr() and strrchr() are already present in the kernel, but with less
popular names. Hence:

• comment current index() and rindex() functions, as these serve the same functionality as, respectively, strchr() and strrchr() from userland;
• add inlined version of strchr() and strrchr(), as we tend to use them more often;
• remove str[r]chr() definitions from ZFS code;

Reviewed by: pjd
Approved by: cognet (mentor)