libc: upgrade some libc functions to _FORTIFY_SOURCE=2

Upgrade the following functions:

* vsnprintf
* vsprintf
* snprintf
* fgets
* strcpy
* strcat
* strncat
* strlcpy
* strlcat
* strlen
* strchr

Change-Id: Icc036fc7f0bb317e05f7c051617887a1601271aa
libc: upgrade strrchr to FORTIFY_SOURCE=2

Change-Id: I4c34c2ce22c5092c4446dc1ab55f37604c1c223f
Use restrict pointers for various libc functions.

All the cool kids say this is the best thing since sliced bread.
http://cellperformance.beyond3d.com/articles/2006/05/demystifying-the-restrict-keyword.html

For the most part, these changes match what glibc does.

Change-Id: I176268f27f82800162fe5f2515b08d5469ea2dfe